Revert predefined decl tracking.

r158085 added some logic to track predefined declarations.  The main reason we
had predefined declarations in the input was because the __builtin_va_list
declarations were injected into the preprocessor input.  As of r158592 we 
explicitly build the __builtin_va_list declarations.  Therefore the predefined
decl tracking is no longer needed.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158732 91177308-0d34-0410-b5e6-96231b3b80d8