When parsing a constraint-expression, a requires-clause or a
requires-expression, we temporarily increment processing_template_decl
so that we always obtain template trees which we could later reduce via
substitution even when not inside a template.

But incrementing processing_template_decl when we're already inside a
template has the unintended side effect of shifting up the template
parameter levels of a lambda defined inside one of these constructs,
which leads to confusion later during substitution into the lambda.

This patch fixes this issue by incrementing processing_template_decl
during parsing of these constructs only if it is 0.

gcc/cp/ChangeLog:

	PR c++/92652
	PR c++/93698
	PR c++/94128
	* parser.c (cp_parser_requires_clause_expression): Temporarily
	increment processing_template_decl only if it is 0.
	(cp_parser_constraint_expression): Likewise.
	(cp_parser_requires_expression): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/92652
	PR c++/93698
	PR c++/94128
	* g++.dg/cpp2a/concepts-lambda8.C: New test.
	* g++.dg/cpp2a/concepts-lambda9.C: New test.
	* g++.dg/cpp2a/concepts-lambda10.C: New test.

(cherry picked from commit 020d86db8896f088435830595640e6fc21bc64ad)