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

[Aidan Goldfarb via cfe-commits]

================
@@ -4870,14 +4870,16 @@ def note_ovl_candidate_inconsistent_deduction_types : Note<
     "candidate template ignored: deduced values %diff{"
     "of conflicting types for parameter %0 (%1 of type $ vs. %3 of type $)|"
     "%1 and %3 of conflicting types for parameter %0}2,4">;
-def note_ovl_candidate_explicit_arg_mismatch_named : Note<
+def note_ovl_candidate_explicit_arg_mismatch : Note<
     "candidate template ignored: invalid explicitly-specified argument "
-    "for template parameter %0">;
+    "for %ordinal0 template parameter">;
+def note_ovl_candidate_explicit_arg_mismatch_detail : Note<
+    "%select{"
+    "|: expected a type, but got value '%1'"
+    "|: expected constant of type %3 but got type %1"
----------------
AidanGoldfarb wrote:

Updated the diags. I am not thrilled about 
```
// FIXME: This is a hack. We should emit a better message 
// for ill-formed const exprs in >=C++20.
if (qt.getCanonicalType() ==
                  SecondArg.getAsType().getCanonicalType() &&
              __cplusplus <= 201703)
```
but I couldn't find another way to avoid the case. Would be resolved in a future PR, along with better messages for `TemplateTemplateParmDecl` cases. 

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