[SemaExprCXX] Avoid calling isInSystemHeader for invalid source locations

[labath]

Summary

While diagnosing a CXXNewExpr warning, we were calling isInSystemHeader(), which expect to be
called with a valid source location. This causes an assertion failure if the location is unknown.
A quick grep shows it's not without precedent to guard calls to the function with a
"Loc.isValid()".

This fixes a test failure in LLDB, which always creates object with invalid source locations as it
does not (always) have access to the source.

cfe/trunk/lib/Sema/SemaExprCXX.cpp

if (OperatorNew) {
 
     // FIXME: Missing call to CheckFunctionCall or equivalent
   }
 
   // Warn if the type is over-aligned and is being allocated by global operator
   // new.
   if (PlacementArgs.empty() && OperatorNew &&
       (OperatorNew->isImplicit() ||
-       getSourceManager().isInSystemHeader(OperatorNew->getLocStart()))) {
+       (OperatorNew->getLocStart().isValid() &&
+        getSourceManager().isInSystemHeader(OperatorNew->getLocStart())))) {
     if (unsigned Align = Context.getPreferredTypeAlign(AllocType.getTypePtr())){
       unsigned SuitableAlign = Context.getTargetInfo().getSuitableAlign();
       if (Align > SuitableAlign)
         Diag(StartLoc, diag::warn_overaligned_type)
             << AllocType
             << unsigned(Align / Context.getCharWidth())
             << unsigned(SuitableAlign / Context.getCharWidth());
     }