Revert "Replace __FUNCTION__ with __func__, the latter being standard C99/C++11."

This reverts commit r201910.

While __func__ may be standard in C++11, it was only recently added to
MSVC in 2013 CTP, and LLVM supports MSVC 2012.  __FUNCTION__ may not be
standard, but it's *very* portable.

llvm-svn: 201916