This CL refactors the implementation of VkSemaphore objects
in the following way:

- Add the ability to support several external handle types
  concurrently. Before this CL, each platform could support
  a single handle type (e.g. on Linux,
  VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_OPAQUE_FD_BIT only).

  The changes here will allow future CLs to support more than
  one type per platform (e.g. the Linux implementation may
  support OPAQUE_FD_BIT as well as SYNC_FD_BIT at the same
  time).

- Better implementation of temporary imports. In particular,
  the following sequence now works properly:

    1) Create exportable semaphore A.
    2) Export A to an external handle/descriptor.
    3) Signal A.
    4) Temporarily import _another_ handle into A.
    5) A.wait()   // waits on the temporary payload, then discard it.

  Before the CL, A would end up, incorrectly, unsignalled. Because
  the export operation created an External instance that held
  the payload modified in 3), which was then discarded after
  the wait() in 5).

- Improved and consistent handling of errors during import/export
  operations, through the use of templates.

+ Add a technical note in VkSemaphore.h explaining how everything
  works, since there are several subtle points in the spec.

Bug: b/140421736

Change-Id: I9b6935db3238fec7af8e0c81666e2f5c72075756
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39880Tested-by: David Turner <digit@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Kokoro-Presubmit: David Turner <digit@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>