[Build] Enable Swift compiler's batch mode

This leads to faster build times when WMO is off.

<rdar://problem/39262812> SwiftPM should build using multifile mode (batch mode)

Author: aciidgh

Sources/Build/BuildPlan.swift

@@ -323,6 +323,17 @@ public final class SwiftTargetDescription {
     public func compileArguments() -> [String] {
         var args = [String]()
         args += ["-swift-version", swiftVersion.rawValue]
+
+        // Enable batch mode in debug mode.
+        //
+        // Technically, it should be enabled whenever WMO is off but we
+        // don't currently make that distinction in SwiftPM
+        switch buildParameters.configuration {
+        case .debug:
+            args += ["-enable-batch-mode"]
+        case .release: break
+        }
+
         args += buildParameters.toolchain.extraSwiftCFlags
         args += optimizationArguments
         args += ["-j\(SwiftCompilerTool.numThreads)"]

Tests/BuildTests/BuildPlanTests.swift

@@ -78,10 +78,10 @@ final class BuildPlanTests: XCTestCase {
         result.checkTargetsCount(2)
 
         let exe = try result.target(for: "exe").swiftTarget().compileArguments()
-        XCTAssertMatch(exe, ["-swift-version", "3", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
+        XCTAssertMatch(exe, ["-swift-version", "3", "-enable-batch-mode", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
 
         let lib = try result.target(for: "lib").swiftTarget().compileArguments()
-        XCTAssertMatch(lib, ["-swift-version", "3", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
+        XCTAssertMatch(lib, ["-swift-version", "3", "-enable-batch-mode", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
 
       #if os(macOS)
         let linkArguments = [
@@ -334,7 +334,7 @@ final class BuildPlanTests: XCTestCase {
         XCTAssertEqual(lib.moduleMap, AbsolutePath("/path/to/build/debug/lib.build/module.modulemap"))
 
         let exe = try result.target(for: "exe").swiftTarget().compileArguments()
-        XCTAssertMatch(exe, ["-swift-version", "3", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG","-Xcc", "-fmodule-map-file=/path/to/build/debug/lib.build/module.modulemap", "-I", "/Pkg/Sources/lib/include", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
+        XCTAssertMatch(exe, ["-swift-version", "3", "-enable-batch-mode", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG","-Xcc", "-fmodule-map-file=/path/to/build/debug/lib.build/module.modulemap", "-I", "/Pkg/Sources/lib/include", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
 
       #if os(macOS)
         XCTAssertEqual(try result.buildProduct(for: "exe").linkArguments(), [
@@ -370,10 +370,10 @@ final class BuildPlanTests: XCTestCase {
       #endif
 
         let foo = try result.target(for: "Foo").swiftTarget().compileArguments()
-        XCTAssertMatch(foo, ["-swift-version", "3", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
+        XCTAssertMatch(foo, ["-swift-version", "3", "-enable-batch-mode", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
 
         let fooTests = try result.target(for: "FooTests").swiftTarget().compileArguments()
-        XCTAssertMatch(fooTests, ["-swift-version", "3", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
+        XCTAssertMatch(fooTests, ["-swift-version", "3", "-enable-batch-mode", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
 
       #if os(macOS)
         XCTAssertEqual(try result.buildProduct(for: "PkgPackageTests").linkArguments(), [
@@ -409,7 +409,7 @@ final class BuildPlanTests: XCTestCase {
         result.checkProductsCount(1)
         result.checkTargetsCount(1)
 
-        XCTAssertMatch(try result.target(for: "exe").swiftTarget().compileArguments(), ["-swift-version", "3", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-Xcc", "-fmodule-map-file=/Clibgit/module.modulemap", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
+        XCTAssertMatch(try result.target(for: "exe").swiftTarget().compileArguments(), ["-swift-version", "3", "-enable-batch-mode", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-Xcc", "-fmodule-map-file=/Clibgit/module.modulemap", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
 
       #if os(macOS)
         XCTAssertEqual(try result.buildProduct(for: "exe").linkArguments(), [
@@ -550,10 +550,10 @@ final class BuildPlanTests: XCTestCase {
         result.checkTargetsCount(2)
 
         let exe = try result.target(for: "exe").swiftTarget().compileArguments()
-        XCTAssertMatch(exe, ["-swift-version", "4", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
+        XCTAssertMatch(exe, ["-swift-version", "4", "-enable-batch-mode", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
 
         let lib = try result.target(for: "lib").swiftTarget().compileArguments()
-        XCTAssertMatch(lib, ["-swift-version", "4", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
+        XCTAssertMatch(lib, ["-swift-version", "4", "-enable-batch-mode", "-Onone", "-g", "-enable-testing", .equal(j), "-DSWIFT_PACKAGE", "-DDEBUG", "-module-cache-path", "/path/to/build/debug/ModuleCache", .anySequence])
 
         #if os(macOS)
             let linkArguments = [