[MC] Prevent out of order HashDirective lexing in AsmLexer.

To lex hash directives we peek ahead to find component tokens, create a
unified token, and unlex the peeked tokens so the parser does not need
to parse the tokens then. Make sure we do not to lex another hash
directive during peek operation.

This fixes PR28921.

Reviewers: rnk, loladiro

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D24839

llvm-svn: 282992