Ignore Lexing errors in macro body definitions

Do not issue lexing errors found during the parsing of macro body
definitions and parseIdentifier function in AsmParser. This changes the
Parser to not issue a lexing error when we reach an error, but rather
when it is consumed allowing us time to examine and recover from an
error.

As a result, of this, we stop issuing a both lexing error and a parsing
error in floating-literals test. Minor tweak to parseDirectiveRealValue
to favor more meaningful lexing error over less helpful parse error.

Reviewers: rnk, majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20535

llvm-svn: 271542