[DWARFv5] Tolerate files not all having an MD5 checksum.

In some cases, for example when compiling a preprocessed file, the
front-end is not able to provide an MD5 checksum for all files. When
that happens, omit the MD5 checksums from the final DWARF, because
DWARF doesn't have a way to indicate that some but not all files have
a checksum.

When assembling a .s file, and some but not all .file directives
provide an MD5 checksum, issue a warning and don't emit MD5 into the
DWARF.

Fixes PR37623.

Differential Revision: https://reviews.llvm.org/D48135

llvm-svn: 334710