Make ASTContext explicitly keep track of the declaration for the C

FILE type, rather than using name lookup to find FILE within the
translation unit. Within precompiled headers, FILE is treated as yet
another "special type" (like __builtin_va_list).

This change should provide a performance improvement (not verified),
since the lookup into the translation unit declaration 
forces the (otherwise unneeded) construction of a large hash table.
More importantly, with precompiled headers, the construction
of that table requires deserializing most of the top-level
declarations from the precompiled header, which are then unused.

Fixes PR 4509.

llvm-svn: 74911

clang/lib/Frontend/PCHWriter.cpp

@@ -1892,6 +1892,7 @@ void PCHWriter::WritePCH(Sema &SemaRef, MemorizeStatCalls *StatCalls,
   AddTypeRef(Context.getObjCClassType(), Record);
   AddTypeRef(Context.getRawCFConstantStringType(), Record);
   AddTypeRef(Context.getRawObjCFastEnumerationStateType(), Record);
+  AddTypeRef(Context.getFILEType(), Record);
   Stream.EmitRecord(pch::SPECIAL_TYPES, Record);
 
   // Write the record containing external, unnamed definitions.