Remove the redundant DeclCtx field in SILFunction.

In all cases the DeclCtx field was supposed to be initialized from the
SILLocation of the function, so we can save one pointer per
SILFunction.

There is one test case change where a different (more precise)
diagnostic is being generated after this change.

lib/SILOptimizer/Transforms/AllocBoxToStack.cpp

@@ -591,7 +591,6 @@ SILFunction *PromotedParamCloner::initCloned(SILFunction *Orig,
   if (Orig->hasUnqualifiedOwnership()) {
     Fn->setUnqualifiedOwnership();
   }
-  Fn->setDeclCtx(Orig->getDeclContext());
   return Fn;
 }