c-family: Avoid ICE on va_arg [PR99324]

c-family: Avoid ICE on va_arg [PR99324]

build_va_arg calls the middle-end mark_addressable, which e.g. requires that
cfun is non-NULL.  The following patch calls instead c_common_mark_addressable_vec
which is the c-family variant similarly to the FE c_mark_addressable and
cxx_mark_addressable, except that it doesn't error on addresses of register
variables.  As the taking of the address is artificial for the .VA_ARG
ifn and when that is lowered goes away, it is similar case to the vector
subscripting for which c_common_mark_addressable_vec has been added.

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

	PR c/99324
	* c-common.c (build_va_arg): Call c_common_mark_addressable_vec
	instead of mark_addressable.  Fix a comment typo -
	neutrallly -> neutrally.

	* gcc.c-torture/compile/pr99324.c: New test.