c++/modules: reduce lazy loading recursion

It turns out that with modules we can call mangle_decl recursively
which is bad because the global mangling state isn't recursion aware.
The recursion happens from write_closure_type_name, which calls
lambda_function, which performs name lookup, which can trigger lazy
loading, which can call maybe_clone_body for a newly loaded cdtor, which
can inspect DECL_ASSEMBLER_NAME, which enters mangling.  This was
observed when using fmtlib as a module with trunk and it leads to a
bogus "mangling conflicts with a previous mangle error" followed by an
ICE from cdtor_comdat_group due to a mangling mismatch.

This patch fixes this by sidestepping lazy loading when performing the
op() lookup in lambda_function, so that we don't accidentally end up
entering mangling recursively.  This should be safe since the lazy load
should still get triggered by some other name lookup.

In passing it was noticed that lazy loading can get excessively recursive
ultimately due to the name lookups performed from check_local_shadow,
which may trigger lazy loading, which cause us to instantiate/clone
things, which end up calling check_local_shadow.  This patch mitigates
this by implementating an optimization suggested by Jason:

> I think we shouldn't bother with check_local_shadow in a clone or
> instantiation, only when actually parsing.

This reduces the maximum depth of lazy loading recursion for a simple
modular Hello World from ~115 to ~12.

gcc/cp/ChangeLog:

	* lambda.cc (lambda_function): Call get_class_binding_direct
	instead of lookup_member to sidestep lazy loading.
	* name-lookup.cc (check_local_shadow): Punt if we're in a
	function context that's not actual parsing.

Reviewed-by: Jason Merill <jason@redhat.com>