[clang] [clang][Sema] Improve template argument deduction diagnostic (PR #122754)

[Aidan Goldfarb via cfe-commits]

================
@@ -11714,13 +11714,37 @@ static void DiagnoseBadDeduction(Sema &S, NamedDecl *Found, Decl *Templated,
     return;
   }
 
-  case TemplateDeductionResult::InvalidExplicitArguments:
+  case TemplateDeductionResult::InvalidExplicitArguments: {
     assert(ParamD && "no parameter found for invalid explicit arguments");
-    if (ParamD->getDeclName())
-      S.Diag(Templated->getLocation(),
-             diag::note_ovl_candidate_explicit_arg_mismatch_named)
-          << ParamD->getDeclName();
-    else {
+    if (ParamD->getDeclName()) {
----------------
AidanGoldfarb wrote:

Would it be redundant to print the index in all cases, even when we can point to it with SourceRange? Would we prefer a message like: 
```
source.cpp:44:5: error: no matching function for call to 'case1'
   44 |     case1<42>(42);
source.cpp:41:6: note: candidate template ignored: invalid explicitly-specified template argument: expected a type, but got value '42' (of type 'int')
   40 | template <typename T>
      |           ~~~~~~~~~~
   41 | void case1(T value) {}
      |      ^
```
(The above is what I have implemented and am planning to push)
over:

```
source.cpp:44:5: error: no matching function for call to 'case1'
   44 |     case1<42>(42);
source.cpp:41:6: note: candidate template ignored: invalid explicitly-specified argument for 1st template parameter: expected a type, but got value '42' (of type 'int')
   40 | template <typename T>
      |           ~~~~~~~~~~
   41 | void case1(T value) {}
      |      ^
```

Although perhaps obvious, I am asking because `candidate template ignored: invalid explicitly-specified template argument: expected a type, but got value '42' (of type 'int')` on its own doesn't give us any location information.

https://github.com/llvm/llvm-project/pull/122754