When importing external buffers, Vulkan ICDs could choose to
import the memory into a memoryType that doesn't support the
VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT property. Account for this
possibility.

Bug: angleproject:5073
Bug: angleproject:5909
Change-Id: Ied063b38fa48d0c8508c4aaca9214cc526f393ad
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2783669
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>