c-family: -Wsequence-point and COMPONENT_REF [PR107163]

c-family: -Wsequence-point and COMPONENT_REF [PR107163]

The patch for PR91415 fixed -Wsequence-point to treat shifts and ARRAY_REF
as sequenced in C++17, and COMPONENT_REF as well.  But this is unnecessary
for COMPONENT_REF, since the RHS is just a FIELD_DECL with no actual
evaluation, and in this testcase handling COMPONENT_REF as sequenced blows
up fast in a deep inheritance tree.  Instead, look through it.

	PR c++/107163

gcc/c-family/ChangeLog:

	* c-common.cc (verify_tree): Don't use sequenced handling
	for COMPONENT_REF.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wsequence-point-5.C: New test.