Add build time flag to control enabling SPIRV-Cross reflection

This adds a flag to enable using the reflection compiler from
SPIRV-Cross, and also updates the build rules to make sure that the
GLSL compiler is also present when using reflection, since the
reflection compiler subclassess the GLSL compiler.

BUG=dawn:231

Change-Id: I4e227cb955a3bb794d906dabdadffdaca001352b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/11920
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

scripts/dawn_features.gni

@@ -37,4 +37,11 @@ declare_args() {
 
   # Enables the compilation of Dawn's Vulkan backend
   dawn_enable_vulkan = is_linux || is_win || is_fuchsia
+
+  # Enable use of reflection compiler in spirv-cross. This is needed
+  # if performing reflection on systems that the platform language
+  # shader is SPIR-V, since there isn't an instance of the
+  # GLSL/HLSL/MSL compiler. This implicitly pulls in the GLSL
+  # compiler, since it is a sub-class of if.
+  dawn_enable_cross_reflection = false
 }

third_party/BUILD.gn

@@ -62,11 +62,10 @@ static_library("spirv_cross") {
     "${spirv_cross_dir}/spirv_cross_parsed_ir.hpp",
     "${spirv_cross_dir}/spirv_parser.cpp",
     "${spirv_cross_dir}/spirv_parser.hpp",
-    "${spirv_cross_dir}/spirv_reflect.cpp",
-    "${spirv_cross_dir}/spirv_reflect.hpp",
   ]
 
-  need_glsl_cross = dawn_enable_opengl
+  need_glsl_cross = dawn_enable_opengl || dawn_enable_cross_reflection
+  need_reflection_cross = dawn_enable_cross_reflection
 
   if (dawn_enable_d3d12) {
     sources += [
@@ -90,6 +89,13 @@ static_library("spirv_cross") {
       "${spirv_cross_dir}/spirv_glsl.hpp",
     ]
   }
+
+  if (need_reflection_cross) {
+    sources += [
+      "${spirv_cross_dir}/spirv_reflect.cpp",
+      "${spirv_cross_dir}/spirv_reflect.hpp",
+    ]
+  }
 }
 
 static_library("spirv_cross_full_for_fuzzers") {