c++: Allow GNU attributes before lambda -> [PR90333]

c++: Allow GNU attributes before lambda -> [PR90333]

In my 9.3/10 patch for 90333 I allowed attributes between [] and (), and
after the trailing return type, but not in the place that GCC 8 expected
them, and we've gotten several bug reports about that.  So let's allow them
there, as well.

gcc/cp/ChangeLog:

	PR c++/90333
	* parser.c (cp_parser_lambda_declarator_opt): Accept GNU attributes
	between () and ->.

gcc/testsuite/ChangeLog:

	PR c++/90333
	* g++.dg/ext/attr-lambda3.C: New test.