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

[Aidan Goldfarb via cfe-commits]

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@@ -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"
+    "|: could not convert '%1' from %2 to %3}0">;
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AidanGoldfarb wrote:

Noted, thank you. I definitely want to dig into the issue, but I am happy to divert it to another PR per your advice. 

I will revert the relevant cases to their original messages (or whatever we determine the new default to be). For the test in `cwg3xx.cpp` that would be `candidate template ignored: invalid explicitly-specified argument for 1st template parameter` (the original unnamed case).

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