SwiftPM complains about @main in non main.swift file when integrated driver is not enabled

One of several different code paths in BuildPlan.swift are taken depending on whether the integrated Swift driver is enabled, whether explicit modules are enabled, etc.

In the non-integrated-driver case, the code path for adding a `-parse-as-library` when necessary wasn't taken.  This moves that code to a more central place, and extends a unit test to test both single and multiple file Swift targets, which seems to trigger the difference in code paths.

rdar://78080245

Author: abertelrud

Fixtures/Miscellaneous/AtMainSupport/Package.swift

@@ -1,14 +1,16 @@
-// swift-tools-version:5.4
+// swift-tools-version:5.5
 import PackageDescription
 
 let package = Package(
     name: "AtMainSupport",
     products: [
-        .executable(name: "ClangExec", targets: ["ClangExec"]),
-        .executable(name: "SwiftExec", targets: ["SwiftExec"]),
+        .executable(name: "ClangExecSingleFile", targets: ["ClangExecSingleFile"]),
+        .executable(name: "SwiftExecSingleFile", targets: ["SwiftExecSingleFile"]),
+        .executable(name: "SwiftExecMultiFile", targets: ["SwiftExecMultiFile"]),
     ],
     targets: [
-        .executableTarget(name: "ClangExec"),
-        .executableTarget(name: "SwiftExec"),
+        .executableTarget(name: "ClangExecSingleFile"),
+        .executableTarget(name: "SwiftExecSingleFile"),
+        .executableTarget(name: "SwiftExecMultiFile"),
     ]
-)
+)

Fixtures/Miscellaneous/AtMainSupport/Sources/ClangExec/NotMain.c renamed to Fixtures/Miscellaneous/AtMainSupport/Sources/ClangExecSingleFile/NotMain.c

@@ -0,0 +1,6 @@
+@main 
+struct MyProgram {
+    static func main() {
+        print("Hello, Swift.")
+    }
+}

Fixtures/Miscellaneous/AtMainSupport/Sources/SwiftExec/NotMain.swift renamed to Fixtures/Miscellaneous/AtMainSupport/Sources/SwiftExecMultiFile/NotMain.swift

@@ -757,6 +757,11 @@ public final class SwiftTargetBuildDescription {
                 }
             }
         }
+        
+        // If the target needs to be parsed without any special semantics involving "main.swift", do so now.
+        if self.needsToBeParsedAsLibrary {
+            args += ["-parse-as-library"]
+        }
 
         // Only add the build path to the framework search path if there are binary frameworks to link against.
         if !libraryBinaryPaths.isEmpty {
@@ -809,10 +814,6 @@ public final class SwiftTargetBuildDescription {
         // FIXME: Eliminate side effect.
         result.append(try writeOutputFileMap().pathString)
 
-        if self.needsToBeParsedAsLibrary {
-            result.append("-parse-as-library")
-        }
-
         if buildParameters.useWholeModuleOptimization {
             result.append("-whole-module-optimization")
             result.append("-num-threads")
@@ -849,10 +850,6 @@ public final class SwiftTargetBuildDescription {
         result.append("-experimental-skip-non-inlinable-function-bodies")
         result.append("-force-single-frontend-invocation")
 
-        if self.needsToBeParsedAsLibrary {
-            result.append("-parse-as-library")
-        }
-
         // FIXME: Handle WMO
 
         for source in target.sources.paths {
@@ -896,9 +893,6 @@ public final class SwiftTargetBuildDescription {
         // FIXME: Eliminate side effect.
         result.append(try writeOutputFileMap().pathString)
 
-        if self.needsToBeParsedAsLibrary {
-            result.append("-parse-as-library")
-        }
         // FIXME: Handle WMO
 
         result.append("-c")

Fixtures/Miscellaneous/AtMainSupport/Sources/SwiftExec/OtherFile.swift renamed to Fixtures/Miscellaneous/AtMainSupport/Sources/SwiftExecMultiFile/OtherFile.swift

@@ -176,15 +176,22 @@ final class BuildToolTests: XCTestCase {
         fixture(name: "Miscellaneous/AtMainSupport") { path in
             let fullPath = resolveSymlinks(path)
             do {
-                let result = try build(["--product", "ClangExec"], packagePath: fullPath)
-                XCTAssert(result.binContents.contains("ClangExec"))
+                let result = try build(["--product", "ClangExecSingleFile"], packagePath: fullPath)
+                XCTAssert(result.binContents.contains("ClangExecSingleFile"))
             } catch SwiftPMProductError.executionFailure(_, let stdout, let stderr) {
                 XCTFail(stdout + "\n" + stderr)
             }
 
             do {
-                let result = try build(["--product", "SwiftExec"], packagePath: fullPath)
-                XCTAssert(result.binContents.contains("SwiftExec"))
+                let result = try build(["--product", "SwiftExecSingleFile"], packagePath: fullPath)
+                XCTAssert(result.binContents.contains("SwiftExecSingleFile"))
+            } catch SwiftPMProductError.executionFailure(_, let stdout, let stderr) {
+                XCTFail(stdout + "\n" + stderr)
+            }
+
+            do {
+                let result = try build(["--product", "SwiftExecMultiFile"], packagePath: fullPath)
+                XCTAssert(result.binContents.contains("SwiftExecMultiFile"))
             } catch SwiftPMProductError.executionFailure(_, let stdout, let stderr) {
                 XCTFail(stdout + "\n" + stderr)
             }