[MC] Cleanup Error Handling in AsmParser

Add parseToken and compatriot functions to stitch error checks in
straight linear code. As part of this fix some erronous handling of
directives where the EndOfStatement token either was not checked or
Lexed on termination.

Reviewers: rnk, majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D22312

llvm-svn: 275795