[clang][Parse] properly parse asm-qualifiers, asm inline

Summary:
The parsing of GNU C extended asm statements was a little brittle and
had a few issues:
- It was using Parse::ParseTypeQualifierListOpt to parse the `volatile`
  qualifier.  That parser is really meant for TypeQualifiers; an asm
  statement doesn't really have a type qualifier. This is still maybe
  nice to have, but not necessary. We now can check for the `volatile`
  token by properly expanding the grammer, rather than abusing
  Parse::ParseTypeQualifierListOpt.
- The parsing of `goto` was position dependent, so `asm goto volatile`
  wouldn't parse. The qualifiers should be position independent to one
  another. Now they are.
- We would warn on duplicate `volatile`, but the parse error for
  duplicate `goto` was a generic parse error and wasn't clear.
- We need to add support for the recent GNU C extension `asm inline`.
  Adding support to the parser with the above issues highlighted the
  need for this refactoring.

Link: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html

Reviewers: aaron.ballman

Reviewed By: aaron.ballman

Subscribers: aheejin, jfb, nathanchance, cfe-commits, echristo, efriedma, rsmith, chandlerc, craig.topper, erichkeane, jyu2, void, srhines

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75563