ClangImporter: make bridging header more portable

`#import` on Windows is problematic at as the default behaviour in C/C++
mode is to be an import of a type library (COM feature).  This can be
adjusted by always enabling Objective-C interop on Windows, but the
runtime is not always built with ObjC interop.

Unfortunately, always using `#include` breaks semantics on Apple where
some places rely on the include-once semantics of `#import`.

For now, control the inclusion technique to be based on whether ObjC
interop is enabled or not.

Author: compnerd

lib/ClangImporter/ClangImporter.cpp

@@ -1352,7 +1352,12 @@ bool ClangImporter::importBridgingHeader(StringRef header, ModuleDecl *adapter,
     return true;
   }
 
-  llvm::SmallString<128> importLine{"#import \""};
+  llvm::SmallString<128> importLine;
+  if (Impl.SwiftContext.LangOpts.EnableObjCInterop)
+    importLine = "#import \"";
+  else
+    importLine = "#include \"";
+
   importLine += header;
   importLine += "\"\n";
 

test/ClangImporter/Inputs/app-bridging-header-to-pch.h

@@ -1,3 +1,10 @@
+
+#ifndef app_bridging_header_to_pch_h
+#define app_bridging_header_to_pch_h
+
 static inline int app_function(int x) {
   return x + 27;
 }
+
+#endif
+

test/ClangImporter/Inputs/chained-unit-test-bridging-header-to-pch.h

@@ -1,5 +1,12 @@
+
+#ifndef chained_unit_test_bridging_header_to_pch_h
+#define chained_unit_test_bridging_header_to_pch_h
+
 #include "app-bridging-header-to-pch.h"
 
 static inline int unit_test_function(int x) {
   return x + 28;
 }
+
+#endif
+