[MC] Defer asm errors to post-statement failure

[MC] Defer asm errors to post-statement failure

Allow errors to be deferred and emitted as part of clean up to simplify
and shorten Assembly parser code. This will allow error messages to be
emitted in helper functions and be modified by the caller which has
better context.

As part of this many minor cleanups to the Parser:

* Unify parser cleanup on error
* Add Workaround for incorrect return values in ParseDirective instances
* Tighten checks on error-signifying return values for parser functions
  and fix in-tree TargetParsers to be more consistent with the changes.
* Fix AArch64 test cases checking for spurious error messages that are
  now fixed.

These changes should be backwards compatible with current Target Parsers
so long as the error status are correctly returned in appropriate
functions.

Reviewers: rnk, majnemer

Subscribers: aemerson, jyknight, llvm-commits

Differential Revision: https://reviews.llvm.org/D24047

llvm-svn: 281249