layers: return collect_* result containers rather than using & out
layers: return collect_* result containers rather than using & out

This is clearer at the call site, should be no slower in C++11+, and
complies with the style guide.

Signed-off-by: Chris Forbes <c3279c50043e18e9442c1ae071cafa0e7a4ca967@google.com>

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools