c++: Fix ICE on PTRMEM_CST in lambda in inline var initializer [PR99790]

The following testcase ICEs (since the addition of inline var support),
because the lambda contains PTRMEM_CST but finish_function is called for the
lambda quite early during parsing it (from finish_lambda_function) when
the containing class is still incomplete.  That means that during
genericization cplus_expand_constant keeps the PTRMEM_CST unmodified, but
later nothing lowers it when the class is finalized.
Using sizeof etc. on the class in such contexts is rejected by both g++ and
clang++, and when the PTRMEM_CST appears e.g. in static var initializers
rather than in functions, we handle it correctly because c_parse_final_cleanups
-> lower_var_init will handle those cplus_expand_constant when all classes
are already finalized.

The following patch fixes it by calling cplus_expand_constant again during
gimplification, as we are now unconditionally unit at a time, I'd think
everything that could be completed will be before we start gimplification.

2021-03-30  Jakub Jelinek  <jakub@redhat.com>

	PR c++/99790
	* cp-gimplify.c (cp_gimplify_expr): Handle PTRMEM_CST.

	* g++.dg/cpp1z/pr99790.C: New test.

(cherry picked from commit 7cdd30b43a63832d6f908b2dd64bd19a0817cd7b)

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