c++: ICE in dependent_type_p with constrained auto [PR97052]

c++: ICE in dependent_type_p with constrained auto [PR97052]

This patch fixes an "unguarded" call to coerce_template_parms in
build_standard_check: processing_template_decl could be zero if we
get here during processing of the first 'auto' parameter of an
abbreviated function template, or if we're processing the type
constraint of a non-templated variable.  In the testcase below, this
leads to an ICE when coerce_template_parms instantiates C's dependent
default template argument.

gcc/cp/ChangeLog:

	PR c++/97052
	* constraint.cc (build_type_constraint): Temporarily increment
	processing_template_decl before calling build_concept_check.
	* pt.c (make_constrained_placeholder_type): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/97052
	* g++.dg/cpp2a/concepts-defarg2.C: New test.