[cxx-interop] Make test discovery compatible with C++ interop

If a Swift module is built with C++ interop enabled, all of its dependencies must also be built with C++ interop enabled.

Swift packages that use test discovery get a synthesized "PackageDiscoveredTests" target which is built by SwiftPM when someone runs `swift test` command. This module is currently always built without C++ interop, which is causing issues for projects that rely on it.

This patch makes sure that the `-cxx-interoperability-mode=` command line flag gets propagated to the test discovery target.

rdar://117078320 / resolves #6990

Author: egorzhdan

Sources/Build/BuildDescription/SwiftTargetBuildDescription.swift

@@ -535,6 +535,22 @@ public final class SwiftTargetBuildDescription {
             args += ["-color-diagnostics"]
         }
 
+        // If this is a generated test discovery target, it might import a test
+        // target that is built with C++ interop enabled. In that case, the test
+        // discovery target must enable C++ interop as well
+        switch testTargetRole {
+        case .discovery:
+            for dependency in try self.target.recursiveTargetDependencies() {
+                let dependencyScope = self.buildParameters.createScope(for: dependency)
+                let dependencySwiftFlags = dependencyScope.evaluate(.OTHER_SWIFT_FLAGS)
+                if let interopModeFlag = dependencySwiftFlags.first(where: { $0.hasPrefix("-cxx-interoperability-mode=") }) {
+                    args += [interopModeFlag]
+                    break
+                }
+            }
+        default: break
+        }
+
         // Add arguments from declared build settings.
         args += try self.buildSettingsFlags()
 

Tests/BuildTests/BuildPlanTests.swift

@@ -3433,6 +3433,7 @@ final class BuildPlanTests: XCTestCase {
             "/A/Sources/cbar/barcpp.cpp",
             "/A/Sources/cbar/bar.c",
             "/A/Sources/cbar/include/bar.h",
+            "/A/Tests/MySwiftTests/test.swift",
 
             "/B/Sources/t1/dep.swift",
             "/B/Sources/t2/dep.swift",
@@ -3483,6 +3484,12 @@ final class BuildPlanTests: XCTestCase {
                         .init(tool: .linker, kind: .unsafeFlags(["-Ilfoo", "-L", "lbar"])),
                     ]
                 ),
+                try TargetDescription(
+                    name: "MySwiftTests", type: .test,
+                    settings: [
+                        .init(tool: .swift, kind: .interoperabilityMode(.Cxx)),
+                    ]
+                ),
             ]
         )
 
@@ -3543,6 +3550,9 @@ final class BuildPlanTests: XCTestCase {
 
             let linkExe = try result.buildProduct(for: "exe").linkArguments()
             XCTAssertMatch(linkExe, [.anySequence, "-lsqlite3", "-llibz", "-Ilfoo", "-L", "lbar", "-g", .end])
+
+            let testDiscovery = try result.target(for: "APackageDiscoveredTests").swiftTarget().compileArguments()
+            XCTAssertMatch(testDiscovery, [.anySequence, "-cxx-interoperability-mode=default"])
         }
 
         // omit frame pointers explicitly set to true