Add SPIR-V support for UseTintGenerator toggle

Turns on Tint generation of SPIR-V if UseTintGenerator is on.
This affects SPIR-V generation for Vulkan, and SPIR-V passed
into SPIRV-Cross to produce GLSL.

It enables DrawTests use_tint_generator for Vulkan/GL where it
should have at least basic support.

Bug: dawn:571
Change-Id: I5df1435bee17572259f5aa3605c4bf19c0136cbc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/32302
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

src/tests/end2end/DrawTests.cpp

@@ -106,5 +106,8 @@ DAWN_INSTANTIATE_TEST(DrawTest,
                       D3D12Backend(),
                       MetalBackend(),
                       OpenGLBackend(),
+                      OpenGLBackend({"use_tint_generator"}),
                       OpenGLESBackend(),
-                      VulkanBackend());
+                      OpenGLESBackend({"use_tint_generator"}),
+                      VulkanBackend(),
+                      VulkanBackend({"use_tint_generator"}));