[ms] [llvm-ml] Add support for line continuations in MASM

[epastor]

Summary

Add support for line continuations (the "backslash operator") in MASM by modifying the Parser's Lex method.

[epastor]

Rebase on parent

[thakis]

test?

[epastor]

Add test. (Sorry, Nico, and thanks!)

[epastor]

Rebase to HEAD

llvm/lib/MC/MCParser/MasmParser.cpp

const AsmToken &MasmParser::Lex() {
 
   // Parse comments here to be deferred until end of next statement.
   while (tok->is(AsmToken::Comment)) {
     if (MAI.preserveAsmComments())
       Out.addExplicitComment(Twine(tok->getString()));
     tok = &Lexer.Lex();
   }
 
+  // Recognize and bypass line continuations.
+  while (tok->is(AsmToken::BackSlash) &&
+         Lexer.peekTok().is(AsmToken::EndOfStatement)) {
+    // Eat both the backslash and the end of statement.
+    Lexer.Lex();
+    tok = &Lexer.Lex();
+  }
+
   if (tok->is(AsmToken::Eof)) {
     // If this is the end of an included file, pop the parent file off the
     // include stack.
     SMLoc ParentIncludeLoc = SrcMgr.getParentIncludeLoc(CurBuffer);
     if (ParentIncludeLoc != SMLoc()) {
       jumpToLoc(ParentIncludeLoc);
       return Lex();
     }