Fix various issues of -ftrivial-auto-var-init=zero with Ada

Fix various issues of -ftrivial-auto-var-init=zero with Ada

This polishes a few rough edges that prevent -ftrivial-auto-var-init=zero
from working in Ada:

  - build_common_builtin_nodes declares BUILT_IN_CLEAR_PADDING with 3
  instead 2 parameters, now gimple_fold_builtin_clear_padding contains
  the assertion:

    gcc_assert (gimple_call_num_args (stmt) == 2)

  This causes gimple_builtin_call_types_compatible_p to always return false
  in Ada (this works in C/C++ because another declaration is used).

  - gimple_add_init_for_auto_var uses EXPR_LOCATION to fetch the location
  of a DECL node, which always returns UNKNOWN_LOCATION.

  - the machinery attempts to initialize Out parameters.

gcc/
	PR middle-end/116933
	* gimplify.cc (gimple_add_init_for_auto_var): Use the correct macro
	to fetch the source location of the variable.
	* tree.cc (common_builtin_nodes): Remove the 3rd parameter in the
	type of BUILT_IN_CLEAR_PADDING.

gcc/ada/
	PR middle-end/116933
	* gcc-interface/decl.cc (gnat_to_gnu_entity) <E_Out_Parameter>: Add
	the "uninitialized" attribute on Out parameters.
	* gcc-interface/utils.cc (gnat_internal_attributes): Add entry for
	the "uninitialized" attribute.
	(handle_uninitialized_attribute): New function.

gcc/testsuite/
	* gnat.dg/auto_var_init.adb: New test.