Module file extensions: pass a Sema through to the extension writer.

Module file extensions are likely to need access to
Sema/Preprocessor/ASTContext, and cannot get it through other
sources.

llvm-svn: 255065

clang/lib/Frontend/TestModuleFileExtension.cpp

@@ -19,6 +19,7 @@ using namespace clang::serialization;
 TestModuleFileExtension::Writer::~Writer() { }
 
 void TestModuleFileExtension::Writer::writeExtensionContents(
+       Sema &SemaRef,
        llvm::BitstreamWriter &Stream) {
   using namespace llvm;