Implement AST/PCH chaining support for macro definitions. Previously,

we would deserialize all of the macro definitions we knew about while
serializing the macro definitions at the end of the AST/PCH file. Even
though we skipped most of them (since they were unchanged), it's still
a performance problem.

Now, we do the standard AST/PCH chaining trick: watch what identifiers
are deserialized as macro names, and consider only those identifiers
(along with macro definitions that have been deserialized/written in
the source) when serializing the preprocessor state.

llvm-svn: 125324