MCNullStreamer: assign file IDs to resolve crashes and errors

Use the MCStreamer base implementations for file ID tracking instead of
overriding them as no-ops.

Avoids assertions when streaming Dwarf debug info, and fixes ASM parsing of loc
and file directives.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211282 91177308-0d34-0410-b5e6-96231b3b80d8