Interned MainFileID within SourceManager. Since SourceManager is referenced by

both Preprocessor and ASTContext, we no longer need to explicitly pass
MainFileID around in function calls that also pass either Preprocessor or
ASTContext. This resulted in some nice cleanups in the ASTConsumers and the
driver.

llvm-svn: 45228

clang/Driver/SerializationTest.cpp

@@ -42,7 +42,7 @@ public:
   
   ~SerializationTest();
 
-  virtual void Initialize(ASTContext& context, unsigned) {
+  virtual void Initialize(ASTContext& context) {
     TU.setContext(&context);
   }