Allow importing opaque file descriptors into memory objects on linux.
Currently this just holds onto the file descriptor rather than calling
vkAllocateMemory immediately. The latter will be easier once we have
support for suballocation (anglebug.com/2162).

Bug: angleproject:3289

Change-Id: Ia80ce07b2a9ec95b9063feb9bfeb24ffe77fa40e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1552028
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>