[Diag] Avoid emitting a redefinition note if no location is available.

[mattd]

Summary

The "previous definition is here" note is not helpful if there is no location information. The note will reference nothing in such a case. This patch first checks to see if there is location data, and if so the note diagnostic is emitted.

This fixes PR15409. The issue in the first comment seems to already be resolved. This patch addresses the second example.

[bruno]

LGTM

lib/Sema/SemaDecl.cpp

if (FNew == FOld && FNewDecLoc.second == FOldDecLoc.second) {
     if (FOld && !HSI.isFileMultipleIncludeGuarded(FOld))
       Diag(Old->getLocation(), diag::note_use_ifdef_guards);
 
     if (EmittedDiag)
       return;
   }
 
   // Redefinition coming from different files or couldn't do better above.
+  if (Old->getLocation().isValid())
     Diag(Old->getLocation(), diag::note_previous_definition);
 }
 
 /// We've just determined that \p Old and \p New both appear to be definitions
 /// of the same variable. Either diagnose or fix the problem.
 bool Sema::checkVarDeclRedefinition(VarDecl *Old, VarDecl *New) {
   if (!hasVisibleDefinition(Old) &&
       (New->getFormalLinkage() == InternalLinkage ||
        New->isInline() ||

test/Sema/redefine_extname.c

 // RUN: %clang_cc1 -triple=x86_64-unknown-linux -Wpragmas -verify %s
 
 // Check that pragma redefine_extname applies to external declarations only.
 #pragma redefine_extname foo_static bar_static
 static int foo_static() { return 1; } // expected-warning {{#pragma redefine_extname is applicable to external C declarations only; not applied to function 'foo_static'}}
 
+unsigned __int128_t; // expected-error {{redefinition of '__int128_t' as different kind of symbol}}