Evaluate arguments of sizeof that are structs of variable size.

Evaluate arguments of sizeof that are structs of variable size.

Evaluate arguments of sizeof for all types of variable size
and not just for VLAs. This fixes some issues related to
[PR29970] where statement expressions need to be evaluated
so that the size is well defined.

2021-08-10  Martin Uecker  <muecker@gwdg.de>

gcc/c/
	PR c/29970
	* c-typeck.c (c_expr_sizeof_expr): Evaluate
	size expressions for structs of variable size.

gcc/testsuite/
	PR c/29970
	* gcc.dg/vla-stexp-1.c: New test.