AsmWriter: Escape string fields in metadata

AsmWriter: Escape string fields in metadata

Properly escape string fields in metadata.  I've added a spot-check with
direct coverage for `MDFile::getFilename()`, but we'll get more coverage
once the hierarchy is moved into place (since this comes up in various
checked-in testcases).

I've replicated the `if` logic using the `ShouldSkipEmpty` flag
(although a follow-up commit is going to change how often this flag is
specified); no NFCI other than escaping the string fields.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230875 91177308-0d34-0410-b5e6-96231b3b80d8