c++: -Wmissing-field-initializers in unevaluated ctx [PR98620]

This PR wants us not to warn about missing field initializers when
the code in question takes places in decltype and similar.  Fixed
thus.

gcc/cp/ChangeLog:

	PR c++/98620
	* typeck2.c (process_init_constructor_record): Don't emit
	-Wmissing-field-initializers warnings in unevaluated contexts.

gcc/testsuite/ChangeLog:

	PR c++/98620
	* g++.dg/warn/Wmissing-field-initializers-2.C: New test.