Add 'gcc/tree.cc:user_omp_clause_code_name' [PR65095]

Add 'gcc/tree.cc:user_omp_clause_code_name' [PR65095]

Re PR65095 "Adapt OpenMP diagnostic messages for OpenACC", move C/C++
front end 'gcc/c-family/c-omp.cc:c_omp_map_clause_name' to generic
'gcc/tree.cc:user_omp_clause_code_name' .  No functional change.

	PR other/65095
	gcc/
	* tree-core.h (user_omp_claus_code_name): Declare function.
	* tree.cc (user_omp_clause_code_name): New function.
	gcc/c/
	* c-typeck.cc (handle_omp_array_sections_1)
	(c_oacc_check_attachments): Call 'user_omp_clause_code_name'
	instead of 'c_omp_map_clause_name'.
	gcc/cp/
	* semantics.cc (handle_omp_array_sections_1)
	(cp_oacc_check_attachments): Call 'user_omp_clause_code_name'
	instead of 'c_omp_map_clause_name'.
	gcc/c-family/
	* c-common.h (c_omp_map_clause_name): Remove.
	* c-omp.cc (c_omp_map_clause_name): Remove.