skip asan-poisoning of discarded vars

skip asan-poisoning of discarded vars

GNAT may create temporaries to hold return values of function calls.
If such a temporary is created as part of a dynamic initializer of a
variable in a unit other than the one being compiled, the initializer
is dropped, including the temporary and its binding block.

Don't issue asan mark calls for such variables, they are gone.

for  gcc/ChangeLog

	* gimplify.c (gimplify_decl_expr): Skip asan marking calls for
	temporaries not seen in binding block, and not about to be
	added as gimple variables.

for  gcc/testsuite/ChangeLog

	* gnat.dg/asan1.adb: New test.
	* gnat.dg/asan1_pkg.ads: New additional source.