Merging r168124 into 3.2 release branch

Fix PR14321, a crash when Clang is built with GCC 4.7 at -O1 or greater.

GCC 4.7 reuses stack slots fairly aggressively, which exposes more temporary
lifetime bugs.

No new test, this was caught by the existing CodeGenCXX/mangle-ms-templates.cpp.

llvm-svn: 168331