[C++11] Don't put empty quotes in static_assert diagnostic.

[Rakete1111]

Summary

This patch removes the empty "" when using static_assert(1 + 1 == 3, ""); in the diagnostic:

main.cpp:1:1: error: static_assert failed
static_assert(1 + 1 == 3, "");
^             ~~~~~~~~~~

[kimgr]

Why? It seems easier to me to map back if the diagnostic mirrors the code as-written.

[Rakete1111]

@kimgr Well, mostly because they bother me a bit, don't know what others think though. I just thought it would be nice if they didn't appear, mainly because there is no need to show empty quotes in the error message. Hmm, you have a point though... Didn't think of that. Thanks +1

lib/Sema/SemaDeclCXX.cpp

                                          Expr *AssertExpr,
                                          Expr *AssertMessageExpr,
                                          SourceLocation RParenLoc) {
-  StringLiteral *AssertMessage =
-      AssertMessageExpr ? cast<StringLiteral>(AssertMessageExpr) : nullptr;
+  auto *AssertMessage = cast_or_null<StringLiteral>(AssertMessageExpr);
+  if (AssertMessage && !AssertMessage->getLength())
+    AssertMessage = nullptr;
 
   if (DiagnoseUnexpandedParameterPack(AssertExpr, UPPC_StaticAssertExpression))
     return nullptr;