MC: Clean up error paths in AsmParser::parseMacroArgument

Use an RAII object Instead of inserting a call to
AsmLexer::setSkipSpace(true) in all error paths.

No functional change.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200358 91177308-0d34-0410-b5e6-96231b3b80d8