[MCAsmParser] Move AltMacroMode tracking out of MCAsmLexer

The Lexer doesn't use this state itself. It is only set and used by AsmParser so it seems like it should just be part of AsmParser.

Differential Revision: https://reviews.llvm.org/D52515

llvm-svn: 343027
diff --git a/llvm/lib/MC/MCParser/MCAsmLexer.cpp b/llvm/lib/MC/MCParser/MCAsmLexer.cpp
index 75cd318..1ed0568 100644
--- a/llvm/lib/MC/MCParser/MCAsmLexer.cpp
+++ b/llvm/lib/MC/MCParser/MCAsmLexer.cpp
@@ -15,7 +15,7 @@
 
 using namespace llvm;
 
-MCAsmLexer::MCAsmLexer() : AltMacroMode(false) {
+MCAsmLexer::MCAsmLexer() {
   CurTok.emplace_back(AsmToken::Space, StringRef());
 }