Reland "Vulkan: Fallback to XCB for Xlib surfaces"

This is a reland of fb0bf70459

Reland after making libx11-xcb dynamically loaded since it isn't present
on all Linux deployment targets of Chromium. Also includes a couple of
additional cosmetic changes to d3d12/PlatformFunctions noticed while
looking at it for inspiration.

Original change's description:
> Vulkan: Fallback to XCB for Xlib surfaces
>
> Chromium builds the Vulkan loader without support Xlib (because it
> prefers XCB) which caused Xlib wgpu::SwapChain creation to fail on the
> Vulkan backend.
>
> This CL adds a fallback to use VK_KHR_xcb_surface if VK_KHR_xlib_surface
> isn't present.
>
> Bug: dawn:662
> Change-Id: I0e0128ee6b5c75da03998dbae231d17e48bacc81
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/41180
> Reviewed-by: Austin Eng <enga@chromium.org>
> Commit-Queue: Austin Eng <enga@chromium.org>
> Auto-Submit: Corentin Wallez <cwallez@chromium.org>

Bug: dawn:662
Change-Id: I617fcd1059dddfa05c29ac20d77f891ca6962342
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/41380
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Jiawei Shao <jiawei.shao@intel.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: Corentin Wallez <cwallez@chromium.org>

src/common/vulkan_platform.h

@@ -162,6 +162,7 @@ namespace dawn_native { namespace vulkan {
 
 #if defined(DAWN_USE_X11)
 #    define VK_USE_PLATFORM_XLIB_KHR
+#    define VK_USE_PLATFORM_XCB_KHR
 #    include "common/xlib_with_undefs.h"
 #endif  // defined(DAWN_USE_X11)
 

src/common/xlib_with_undefs.h

@@ -26,6 +26,10 @@
 // interface.
 #include <X11/Xlib.h>
 
+// Xlib-xcb.h technically includes Xlib.h but we separate the includes to make it more clear what
+// the problem is if one of these two includes fail.
+#include <X11/Xlib-xcb.h>
+
 #undef Success
 #undef None
 #undef Always