c++: ICE with unnamed tparm and concept [PR103408]

c++: ICE with unnamed tparm and concept [PR103408]

Here we crash when issuing the "constraint C has type T, not bool"
error, because pp_cxx_parameter_mapping wasn't prepared to see an
anonymous template parameter.  With this patch we print

error: constraint 'auto(<lambda>) [with <unnamed> = 0]' has type '<lambda()>', not 'bool'

The "<unnamed>" is what this patch adds.

	PR c++/103408

gcc/cp/ChangeLog:

	* cxx-pretty-print.c (pp_cxx_parameter_mapping): Print "<unnamed>"
	rather than crash on an unnamed template parameter.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/concepts-err1.C: New test.