c: ignore initializers for elements of variable-size types [PR93577]

c: ignore initializers for elements of variable-size types [PR93577]

Bug 93577, apparently a regression (although it isn't very clear to me
exactly when it was introduced; tests I made with various past
compilers produced inconclusive results, including e.g. ICEs appearing
with 64-bit-host compilers for some versions but not 32-bit-host
compilers for the same versions) is an C front-end tree-checking ICE
processing initializers for structs using the VLA-in-struct extension.
There is an error for such initializers, but other processing that
still takes place for them results in the ICE.

This patch ensures that processing of initializers for variable-size
types stops earlier to avoid the code that results in the ICE (and
ensures it stops earlier for error_mark_node to avoid ICEs in the
check for variable-size types), adjusts the conditions for the "empty
scalar initializer" diagnostic to avoid consequent excess errors in
the case of a bad type name, and adds tests for a few variations on
what such initializers might look like, as well as tests for cases
identified from ICEs seen with an earlier version of this patch.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

	PR c/93577
gcc/c:
	* c-typeck.c (pop_init_level): Do not diagnose initializers as
	empty when initialized type is error_mark_node.
	(set_designator, process_init_element): Ignore initializers for
	elements of a variable-size type or of error_mark_node.

gcc/testsuite:
	* gcc.dg/pr93577-1.c, gcc.dg/pr93577-2.c, gcc.dg/pr93577-3.c,
	gcc.dg/pr93577-4.c, gcc.dg/pr93577-5.c, gcc.dg/pr93577-6.c: New
	tests.
	* gcc.dg/vla-init-1.c: Expect fewer errors about VLA initializer.