[SafeStack] Set debug location for calls to __safestack_pointer_address.

Otherwise, the debug info is incorrect.  On its own, this is mostly
harmless, but the safe-stack also later inlines the call to
__safestack_pointer_address, which leads to debug info with the wrong
scope, which eventually causes an assertion failure (and incorrect debug
info in release mode).

Differential Revision: https://reviews.llvm.org/D51075

llvm-svn: 340651