MCParser: add a single token lookahead

Some of the more complex directive and macro handling for GAS compatibility
requires lookahead.  Add a single token lookahead in the MCAsmLexer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201058 91177308-0d34-0410-b5e6-96231b3b80d8