c: Handle REALPART_EXPR and IMAGPART_EXPR in fold_offsetof [PR105555]

c: Handle REALPART_EXPR and IMAGPART_EXPR in fold_offsetof [PR105555]

In some cases the C front-end calls into fold_offsetof to fold
an address but that does not handle REALPART_EXPR nor IMAGPART_EXPR so
gcc produces an internal compiler error. For offsetof, REALPART_EXPR/IMAGPART_EXPR
won't show up which is why they were not there before.

Bootstrapped and tested on x86_64-linux-gnu.

	PR c/105555

gcc/c-family/ChangeLog:

	* c-common.cc (fold_offsetof): Handle REALPART_EXPR
	and IMAGPART_EXPR.

gcc/testsuite/ChangeLog:

	* gcc.dg/complex-10.c: New test.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>