[clang] crashes when a concept is specialised as a class template

[cjdb]

Problem
The following program crashes Clang and can be replicated using clang++ -std=c++20.

template<class T>
concept C = true;

template<class T>
class C<int> {};

Clang version
Confirmed on a4c8fb9.
Expected behaviour
Print an error message similar to the following.

/tmp/error.cpp:4:1: error: we attempted to specialise 'C' as a class template, but 'C' is a concept
class C<int>
      ^~~~~~
/tmp/error.cpp:1:1: note: first declared here
concept C = true;
        ^~~~~~~~

[llvmbot]

@llvm/issue-subscribers-c-20

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[tbaederr]

This "just" fails during diagnostic emission.

This diff fixes the crash:

diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
index 18adb21e2be0..1449796ac212 100644
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -775,7 +775,8 @@ def warn_cxx14_compat_template_template_param_typename : Warning<
 def err_template_spec_syntax_non_template : Error<
   "identifier followed by '<' indicates a class template specialization but "
   "%0 %select{does not refer to a template|refers to a function template|"
-  "<unused>|refers to a variable template|<unused>|refers to a concept}1">;
+  "<unused>|refers to a variable template|<unused>|refers to a concept|"
+  "is a concept template}1">;
 def err_id_after_template_in_nested_name_spec : Error<
   "expected template name after 'template' keyword in nested name specifier">;
 def err_unexpected_template_in_unqualified_id : Error<

[tbaederr]

https://reviews.llvm.org/D132821