[ClangImporter][ModuleWrap] Turn off libc warnings.

`swift-modulewrap` uses the `ClangImporter` to obtain a module loader,
but it doesn't take an SDK argument (nor does anything bother to pass
one), which means that when cross-compiling you get warnings about not
being able to find the C library.

Suppress the warning by telling the `ClangImporter` that we don't care
about the C library here.

rdar://115918181

Author: al45tair

include/swift/ClangImporter/ClangImporter.h

@@ -188,7 +188,8 @@ class ClangImporter final : public ClangModuleLoader {
   static std::unique_ptr<ClangImporter>
   create(ASTContext &ctx,
          std::string swiftPCHHash = "", DependencyTracker *tracker = nullptr,
-         DWARFImporterDelegate *dwarfImporterDelegate = nullptr);
+         DWARFImporterDelegate *dwarfImporterDelegate = nullptr,
+         bool ignoreFileMapping = false);
 
   static std::vector<std::string>
   getClangDriverArguments(ASTContext &ctx, bool ignoreClangTarget = false);

lib/ClangImporter/ClangImporter.cpp

@@ -1275,7 +1275,8 @@ std::unique_ptr<clang::CompilerInvocation> ClangImporter::createClangInvocation(
 std::unique_ptr<ClangImporter>
 ClangImporter::create(ASTContext &ctx,
                       std::string swiftPCHHash, DependencyTracker *tracker,
-                      DWARFImporterDelegate *dwarfImporterDelegate) {
+                      DWARFImporterDelegate *dwarfImporterDelegate,
+                      bool ignoreFileMapping) {
   std::unique_ptr<ClangImporter> importer{
       new ClangImporter(ctx, tracker, dwarfImporterDelegate)};
   auto &importerOpts = ctx.ClangImporterOpts;
@@ -1296,7 +1297,11 @@ ClangImporter::create(ASTContext &ctx,
   llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS =
       ctx.SourceMgr.getFileSystem();
 
-  auto fileMapping = getClangInvocationFileMapping(ctx);
+  ClangInvocationFileMapping fileMapping;
+
+  if (!ignoreFileMapping)
+    fileMapping = getClangInvocationFileMapping(ctx);
+
   // Avoid creating indirect file system when using include tree.
   if (!ctx.ClangImporterOpts.HasClangIncludeTreeRoot) {
     // Wrap Swift's FS to allow Clang to override the working directory

lib/DriverTool/modulewrap_main.cpp

@@ -196,8 +196,11 @@ int modulewrap_main(ArrayRef<const char *> Args, const char *Argv0,
       llvm::makeIntrusiveRefCnt<llvm::vfs::OnDiskOutputBackend>());
   registerParseRequestFunctions(ASTCtx.evaluator);
   registerTypeCheckerRequestFunctions(ASTCtx.evaluator);
-  
-  ASTCtx.addModuleLoader(ClangImporter::create(ASTCtx, ""), true);
+
+  ASTCtx.addModuleLoader(ClangImporter::create(ASTCtx, "",
+                                               nullptr, nullptr,
+                                               true),
+                         true);
   ModuleDecl *M = ModuleDecl::create(ASTCtx.getIdentifier("swiftmodule"), ASTCtx);
   std::unique_ptr<Lowering::TypeConverter> TC(
       new Lowering::TypeConverter(*M, ASTCtx.SILOpts.EnableSILOpaqueValues));