Evaluate type arguments of sizeof that are structs of variable size [PR101838]

Evaluate type arguments of sizeof that are structs of variable size [PR101838]

Evaluate type arguments of sizeof for all types of variable size
and not just for VLAs. This fixes PR101838 and some issues related
to PR29970 where statement expressions need to be evaluated so that
the size is well defined.

2021-08-12  Martin Uecker  <muecker@gwdg.de>

gcc/c/
	PR c/101838
	PR c/29970
	* c-typeck.c (c_expr_sizeof_type): Evaluate
	size expressions for structs of variable size.

gcc/testsuite/
	PR c/101838
	* gcc.dg/vla-stexp-2.c: New test.