[interop][SwiftToCxx] include _SwiftCxxInteroperability.h in the toolchain, relative to clang's resource dir

Author: hyp

lib/PrintAsClang/PrintAsClang.cpp

@@ -105,7 +105,13 @@ static void writePrologue(raw_ostream &out, ASTContext &ctx,
                "#include <cstring>\n";
         out << "#include <stdlib.h>\n";
         out << "#include <new>\n";
-        out << "#if __has_include(<shims/_SwiftCxxInteroperability.h>)\n";
+        // FIXME: Look for the header in the SDK.
+        out << "// Look for the C++ interop support header relative to clang's resource dir:\n";
+        out << "//  '<toolchain>/usr/lib/clang/<version>/include/../../../swift/shims'.\n";
+        out << "#if __has_include(<../../../swift/shims/_SwiftCxxInteroperability.h>)\n";
+        out << "#include <../../../swift/shims/_SwiftCxxInteroperability.h>\n";
+        out << "// Alternatively, allow user to find the header using additional include path into 'swift'.\n";
+        out << "#elif __has_include(<shims/_SwiftCxxInteroperability.h>)\n";
         out << "#include <shims/_SwiftCxxInteroperability.h>\n";
         out << "#endif\n";
       },

test/PrintAsCxx/empty.swift

@@ -31,7 +31,12 @@
 // CHECK-NEXT:  #include <cstring>
 // CHECK-NEXT:  #include <stdlib.h>
 // CHECK-NEXT:  #include <new>
-// CHECK-NEXT:  #if __has_include(<shims/_SwiftCxxInteroperability.h>)
+// CHECK-NEXT:  // Look for the C++ interop support header relative to clang's resource dir:
+// CHECK-NEXT:  // '<toolchain>/usr/lib/clang/<version>/include/../../../swift/shims'.
+// CHECK-NEXT:  #if __has_include(<../../../swift/shims/_SwiftCxxInteroperability.h>)
+// CHECK-NEXT:  #include <../../../swift/shims/_SwiftCxxInteroperability.h>
+// CHECK-NEXT:  // Alternatively, allow user to find the header using additional include path into 'swift'.
+// CHECK-NEXT:  #elif __has_include(<shims/_SwiftCxxInteroperability.h>)
 // CHECK-NEXT:  #include <shims/_SwiftCxxInteroperability.h>
 // CHECK-NEXT:  #endif
 // CHECK-NEXT:  #else