This is a reland of 552f5fcb
There was a missing build dependence causing a build no-op
issue which has now been fixed in the upstream Vulkan Validation
Layer repo.

Primary workaround that need to be re-visted and fixed are:
1. Disabled validation layers on 32-bit Windows
2. Disabled validation layers on 32-bit Android
3. Disabled DebugUtils on Android, using DebugReport instead
4. Using custom VK Loader branch to avoid WDK dependence

Original change's description:
> Vulkan: Use VK repos' internal BUILD.gn files
>
> This retires custom BUILD.gn files for the Vulkan Headers, Tools,
> Validation-Layers, and Loader repos. They now have integrated BUILD.gn
> files so switched ANGLE to use those by default.
> Also Validation Layer, Loader, and Tools repos no longer uses codegen
> so this speeds up the build overall.
> Switched over from old VK_LAYER_LUNARG_standard_validation meta-layer
> to VK_LAYER_KHRONOS_validation unified layer.
>
> This changes includes a temporary workaround to the Vulkan-Loader repo
> to remove its Window's build dependence on WDK. There are plans to
> remove that depenedence in Loader master, but until then we can use
> our custom branch workaround.
> It also includes a custom branch for validation layers that includes
> some build fixes for ANGLE.
> Finally, the layers were crashing on Android when attempting to use
> DebugUtils extension so forcing DebugReport on Android for now.
>
> Bug: angleproject:2449
> Bug: angleproject:3320
> Bug: angleproject:3852
> Bug: angleproject:3924
> Bug: angleproject:3925
> Bug: angleproject:3946
> Bug: chromium:997009
> Bug: chromium:997016
> Change-Id: I48b36acefcc7f2bc930eb72d6fdbc87bca24f833
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1610438
> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Commit-Queue: Tobin Ehlis <tobine@google.com>

Bug: angleproject:2449, angleproject:3320, angleproject:3852
Change-Id: I373ada091a1329db5e304874079428d01ba21d52
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1778302Reviewed-by: Tobin Ehlis <tobine@google.com>
Commit-Queue: Tobin Ehlis <tobine@google.com>