Reapply r151638 and r151641.

The bug that was caught by Apple's internal buildbots was valid and also showed another bug in my implementation.

These are now fixed, with regression tests added to catch them both (not Darwin-specific).

Original log:
====================

Revert r151638 because it causes assertion hit on PCH creation for Cocoa.h

Original log:
---------------------
Correctly track tags and enum members defined in the prototype of a function, and ensure they are properly scoped.

This fixes code such as:

enum e {x, y};
int f(enum {y, x} n) {
 return 0;
}

This finally fixes PR5464 and PR5477.
---------------------

I also reverted r151641 which was enhancement on top of r151638.

====================

llvm-svn: 151712