Fix a crash with assembler source and -g.

llvm-mc or clang with -g normally produces debug info describing the
assembler source itself; however, if that source already contains some
.file/.loc directives, we should instead emit the debug info described
by those directives.  For certain assembler sources seen in the wild
(particularly in the Chrome build) this was causing a crash due to
incorrect assumptions about legal sequences of assembler source text.

Fixes PR38994.

Differential Revision: https://reviews.llvm.org/D63573

llvm-svn: 364039