Revert r153723, and its follow-ups r153728 and r153733.

These patches cause us to miscompile and/or reject code with static
function-local variables in an extern-C context. Previously, we were
papering over this as long as the variables are within the same
translation unit, and had not seen any failures in the wild. We still
need a proper fix, which involves mangling static locals inside of an
extern-C block (as GCC already does), but this patch causes pretty
widespread regressions. Firefox, and many other applications no longer
build.

Lots of test cases have been posted to the list in response to this
commit, so there should be no problem reproducing the issues.

llvm-svn: 153768