Teach the .ll parser to handle named metadata with non-simple names.

Teach the .ll parser to handle named metadata with non-simple names.
Unfortunately we can't follow what the rest of the language does (wrapping it
in double-quotes) because that would cause an ambiguity with metadata strings,
so instead we escape any unusual characters with \xx escaping.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133050 91177308-0d34-0410-b5e6-96231b3b80d8