Fix explicit template parameter reporting for narrowing conversions

[erichkeane]

Summary

I found that explicit template parameters that caused a
narrowing integer conversion resulted in the incorrect parameter
being mentioned in the note (see test attached). This is because
the argument checking code doesn't check to see if it caused
SFINAE errors when checking the arguments, so instead of giving
up on the first error, it continues through the list. This
makes the error reporting pick up the last template param every time.

This patch checks these parameters on each argument and gives up
if there is an error. The result is that only the required amount
of arguments are checked, and that the 'Converted' array contains
only the successful arguments before the first failure, as the
calls seem to all expect.

[erichkeane]

Did Review-after-commit, in retrospect this is a pretty tiny change.

lib/Sema/SemaTemplate.cpp

if (NonTypeTemplateParmDecl *NTTP =dyn_cast<NonTypeTemplateParmDecl>(Param)) {
     }
 
     switch (Arg.getArgument().getKind()) {
     case TemplateArgument::Null:
       llvm_unreachable("Should never see a NULL template argument here");
 
     case TemplateArgument::Expression: {
       TemplateArgument Result;
+      unsigned CurSFINAEErrors = NumSFINAEErrors;
       ExprResult Res =
         CheckTemplateArgument(NTTP, NTTPType, Arg.getArgument().getAsExpr(),
                               Result, CTAK);
       if (Res.isInvalid())
         return true;
+      // If the current template argument causes an error, give up now.
+      if (CurSFINAEErrors < NumSFINAEErrors)
+        return true;
 
       // If the resulting expression is new, then use it in place of the
       // old expression in the template argument.
       if (Res.get() != Arg.getArgument().getAsExpr()) {
         TemplateArgument TA(Res.get());
         Arg = TemplateArgumentLoc(TA, Res.get());
       }
 

test/SemaTemplate/temp_arg_nontype_cxx11.cpp

 namespace check_conversion_early {
   struct X {};
   template<int> struct A {};
   template<X &x> struct A<x> {}; // expected-error {{not implicitly convertible}}
 
   struct Y { constexpr operator int() const { return 0; } };
   template<Y &y> struct A<y> {}; // expected-error {{cannot be deduced}} expected-note {{'y'}}
 }
+
+namespace ReportCorrectParam {
+template <int a, unsigned b, int c>
+void TempFunc() {}
+
+void Useage() {
+  //expected-error@+2 {{no matching function}}
+  //expected-note@-4 {{candidate template ignored: invalid explicitly-specified argument for template parameter 'b'}}
+  TempFunc<1, -1, 1>();
+}
+}
+