Merging r276956:

------------------------------------------------------------------------
r276956 | majnemer | 2016-07-27 22:03:22 -0700 (Wed, 27 Jul 2016) | 6 lines

[CodeView] Don't crash on functions without subprograms

A function may have instructions annotated with debug info without
having a subprogram.

This fixes PR28747.
------------------------------------------------------------------------

llvm-svn: 277078