[Ada] Spurious warning on uninitialized entity during code generation

[Ada] Spurious warning on uninitialized entity during code generation

This patch suppresses a spurious warning coming from the GCC backend, on
an aggregate that cannot be built in place and for which a temporary
variable must be created. If the type of the aggregate is a packed
boolean array, the generated code may appear to use an uninitialized
value for a component of the array, when in fact the code simply sets a
single bit of that array.

2018-10-09  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_aggr.adb (Expand_Array_Aggregate): If it is not possible
	to build in place an aggregate with component associations, set
	the Warnings_Off flag on the generated temporary, to prevent
	spurious warnings from the backend when compiling with the
	-Wuninitialized gcc flag.

gcc/testsuite/

	* gnat.dg/warn18.adb: New testcase.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@264960 138bc75d-0d04-0410-961f-82ee72b054a4