c++: Fix up #pragma omp declare {simd,variant} and acc routine parsing

c++: Fix up #pragma omp declare {simd,variant} and acc routine parsing

When parsing default arguments, we need to temporarily clear parser->omp_declare_simd
and parser->oacc_routine, otherwise it can clash with further declarations
inside of e.g. lambdas inside of those default arguments.

2021-08-04  Jakub Jelinek  <jakub@redhat.com>

	PR c++/101759
	* parser.c (cp_parser_default_argument): Temporarily override
	parser->omp_declare_simd and parser->oacc_routine to NULL.

	* g++.dg/gomp/pr101759.C: New test.
	* g++.dg/goacc/pr101759.C: New test.

(cherry picked from commit af31cab04770f7a1a1da069415ab62ca2ef54fc4)