MCAsmParser: full support for gas' '.if{cond} expression' directives

Patch by Janne Grunau!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211218 91177308-0d34-0410-b5e6-96231b3b80d8
MC: do not add comment string to the AsmToken in AsmLexer::LexLineComment

Fixes macros with varargs if the macro instantiation has a trailing comment.

Patch by Janne Grunau!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211219 91177308-0d34-0410-b5e6-96231b3b80d8