[ms] [llvm-ml] Accept /WX to signal that warnings should be fatal.

[epastor]

Summary

Define -fatal-warnings to make warnings fatal, and accept /WX as an ML.EXE compatible alias for it.

Also make sure that if Warning() returns true, we always treat it as an error.

[thakis]

Test?

[epastor]

Rebase on parent

[epastor]

Revised to include fatal warning support, and test both cases

[epastor]

Rebase to HEAD

llvm/lib/MC/MCParser/MasmParser.cpp

 bool MasmParser::parseDirectiveAlign() {
   SMLoc AlignmentLoc = getLexer().getLoc();
   int64_t Alignment;
 
   if (checkForValidSection())
     return addErrorSuffix(" in align directive");
   // Ignore empty 'align' directives.
   if (getTok().is(AsmToken::EndOfStatement)) {
-    Warning(AlignmentLoc, "align directive with no operand is ignored");
-    return parseToken(AsmToken::EndOfStatement);
+    return Warning(AlignmentLoc,
+                   "align directive with no operand is ignored") &&
+           parseToken(AsmToken::EndOfStatement);
   }
   if (parseAbsoluteExpression(Alignment) ||
       parseToken(AsmToken::EndOfStatement))
     return addErrorSuffix(" in align directive");
 
   // Always emit an alignment here even if we thrown an error.
   bool ReturnVal = false;