[clang][CodeGenPGO] Don't use an invalid index when region counts dis…

…agree

If we're using an old instrprof profile and the user passes we can get
Decls with children decl counts not matching the what the profile was
written against. In a particular case I was debugging we have 24 decls
in the AST and 22 decls in the profile. Avoid crashing in this case.

Differential Revision: https://reviews.llvm.org/D149504

clang/lib/CodeGen/CodeGenPGO.h

@@ -114,7 +114,12 @@ class CodeGenPGO {
       return 0;
     if (!haveRegionCounts())
       return 0;
-    return RegionCounts[(*RegionCounterMap)[S]];
+    // With profiles from a differing version of clang we can have mismatched
+    // decl counts. Don't crash in such a case.
+    auto Index = (*RegionCounterMap)[S];
+    if (Index >= RegionCounts.size())
+      return 0;
+    return RegionCounts[Index];
   }
 };