[debuginfo] generate debug info with asm+.file

Summary:
For assembly input files, generate debug info even when the .file
directive is present, provided it does not include a file-number
argument.  Fixes PR38695.

Reviewers: probinson, sidneym

Subscribers: aprantl, hiraditya, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D51315

llvm-svn: 340839