This moves the build configuration into the Vulkan back-end dir.
This should be a little easier to maintain as all Vulkan-related
config is in one place.

Note that this should not interfere with Skia's build as they do
not import the Vulkan back-end sources.

One additional possiblity that this enables is testing other
compile-time permutations of the Vulkan back-end more easily. For
example we could make a simple change to enable compile testing
of the Vulkan back-end with custom command buffers disabled.

Also fixes a few errors affecting less tested configs.

Bug: angleproject:3943
Change-Id: Iaf819936896e4f5d3e6415ed16ab0c940e46cdb6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1829662
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>