[Ada] Spurious error -- "allocation from empty storage pool"

[Ada] Spurious error -- "allocation from empty storage pool"

This patch fixes a bug in which if "pragma Default_Storage_Pool (null);"
is given, then a build-in-place function will get an incorrect error
message "allocation from empty storage pool" even though there is no
such allocation in the source program.

2018-07-31  Bob Duff  <duff@adacore.com>

gcc/ada/

	* sem_res.adb (Resolve_Allocator): Do not complain about the
	implicit allocator that occurs in the expansion of a return
	statement for a build-in-place function.

From-SVN: r263088