Debug info: Apply an empty debug location for global OpenMP destructors.

LLVM really wants a debug location on every inlinable call in a function
with debug info, because it otherwise cannot set up inlining debug info.

This change applies an artificial line 0 debug location (which is how
DWARF marks automatically generated code that has no corresponding
source code) to the .__kmpc_global_dtor_. functions to avoid the
LLVM Verifier complaining.

llvm-svn: 267369