loader: GH370 - re-enable allocator usage

loader: GH370 - re-enable allocator usage

Re-enable the allocator usage in the loader.  Also, fix several
memory leaks.  The leaks were especially noticeable during the Vulkan
conformance object_management.alloc_callback_fail.instance and
object_management.alloc_callback_fail.device tests because the
tests were designed to intentionally fail the Alloc calls.

This change now fully passes the Vulkan CTS
object_management.alloc_callback_fail tests when the loader
uses application-provided Allocators.

Change-Id: I03f2a09bc33259442e02c917a34d78f8937808d8