c++: Handle COMPOUND_EXPRs in ocp_convert [PR94339]

c++: Handle COMPOUND_EXPRs in ocp_convert [PR94339]

My recent change to get_narrower/warnings_for_convert_and_check broke
the following testcase, warnings_for_convert_and_check is upset that
expr is a COMPOUND_EXPR with INTEGER_CST at the rightmost operand, while
result is a COMPOUND_EXPR with a NOP_EXPR of INTEGER_CST at the rightmost
operand, it expects such conversions to be simplified.

The easiest fix seems to be to handle COMPOUND_EXPRs in ocp_convert too,
by converting the rightmost operand and recreating COMPOUND_EXPR(s) if that
changed.

The attr-copy-2.C change is a workaround for PR94346, where we now ICE on
the testcase, while previously we'd ICE only if it contained a comma
expression at the outer level rather than cast of a COMPOUND_EXPR to
something.  I'll defer that to Martin.

2020-03-27  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94339
	* cvt.c (ocp_convert): Handle COMPOUND_EXPR by recursion on the second
	operand and creating a new COMPOUND_EXPR if anything changed.

	* g++.dg/other/pr94339.C: New test.
	* g++.dg/ext/attr-copy-2.C: Comment out failing tests due to PR94346.