c++: Fix up mangling of static lambdas [PR108525]

c++: Fix up mangling of static lambdas [PR108525]

Before the P1169R4 changes, operator () of a lambda was
always a method, so it was fine to pass method_p = 1 unconditionally,
but it isn't always the case, so this patch adds a check for whether
it is a method or nor.

2023-01-25  Jakub Jelinek  <jakub@redhat.com>

	PR c++/108525
	* mangle.cc (write_closure_type_name): Don't assume all
	lambda operator() fns are methods.

	* g++.dg/cpp23/static-operator-call5.C: New test.