c: Fix up -Wunused-but-set-* warnings for _Atomics [PR99588]

c: Fix up -Wunused-but-set-* warnings for _Atomics [PR99588]

As the following testcases show, compared to -D_Atomic= case we have many
-Wunused-but-set-* warning false positives.
When an _Atomic variable/parameter is read, we call mark_exp_read on it in
convert_lvalue_to_rvalue, but build_atomic_assign does not.
For consistency with the non-_Atomic case where we mark_exp_read the lhs
for lhs op= ... but not for lhs = ..., this patch does that too.
But furthermore we need to pattern match the trees emitted by _Atomic store,
so that _Atomic store itself is not marked as being a variable read, but
when the result of the store is used, we mark it.

2021-03-19  Jakub Jelinek  <jakub@redhat.com>

	PR c/99588
	* c-typeck.c (mark_exp_read): Recognize what build_atomic_assign
	with modifycode NOP_EXPR produces and mark the _Atomic var as read
	if found.
	(build_atomic_assign): For modifycode of NOP_EXPR, use COMPOUND_EXPRs
	rather than STATEMENT_LIST.  Otherwise call mark_exp_read on lhs.
	Set TREE_SIDE_EFFECTS on the TARGET_EXPR.

	* gcc.dg/Wunused-var-5.c: New test.
	* gcc.dg/Wunused-var-6.c: New test.

(cherry picked from commit b1fc1f1c4b2e9005c40ed476b067577da2d2ce84)