layers: Convert fundamental type enum to bit field

Some VkFormats can be consumed as multiple types -- specifically,
depth/stencil is either float or uint, depending on which aspect we
consume.

Convert to bitfield, and require intersection rather than equality.
While we're at it, drop out some weird special-casing for unknown
component types-- this is left over from when shader validation was much
less complete.

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