c, c++: Fix up -Wunused-value on COMPLEX_EXPRs [PR97748]

The -Wunused-value warning in both C and C++ FEs (implemented
significantly differently between the two) sees the COMPLEX_EXPRs created
e.g. for complex pre/post increment and many other expressions as useless
and warns about it.

For the C warning implementation, on e.g.
COMPLEX_EXPR < ++REALPART_EXPR <x>, IMAGPART_EXPR <x>>;
would warn even on the IMAGPART_EXPR <x> there alone etc., so what works
is check if we'd warn about both operands of COMPLEX_EXPR and if yes,
warn on the whole COMPLEX_EXPR, otherwise don't warn.

The C++ warning implementation is significantly different and for that one
the only warn if both would be warned about doesn't really work,
we then miss warnings e.g. about
COMPLEX_EXPR <REALPART_EXPR <SAVE_EXPR <x>> + 1.0e+0, IMAGPART_EXPR <SAVE_EXPR <x>>> >>>>>
The patch replaces the warning_at call with call to the c-family
warn_if_unused_value function.

On the testcase which after the initial new tests contains pretty much
everything from gcc.dg/Wunused-value-1.c both approaches seem to work
nicely.

2020-11-10  Jakub Jelinek  <jakub@redhat.com>

	PR c/97748
gcc/c-family/
	* c-common.h (warn_if_unused_value): Add quiet argument defaulted
	to false.
	* c-warn.c (warn_if_unused_value): Likewise.  Pass it down
	recursively and just return true instead of warning if it is true.
	Handle COMPLEX_EXPR.
gcc/cp/
	* cvt.c (convert_to_void): Check (complain & tf_warning) in the outer
	if rather than twice times in the inner one.  Use warn_if_unused_value.
	Formatting fix.
gcc/testsuite/
	* c-c++-common/Wunused-value-1.c: New test.