Convert spvc build flag to a runtime toggle

Also moves some of the spirv_cross code into the main library that was
feature guarded, since spvc requires it.

BUG=dawn:281

Change-Id: I482d1d5a5c851956d3815bad90665c52a1ea15bb
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/13860
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/Toggles.cpp

@@ -82,7 +82,11 @@ namespace dawn_native {
                "versions of Windows prior to build 1809, or when this toggle is turned off, Dawn "
                "will emulate a render pass."}},
              {Toggle::SkipValidation,
-              {"skip_validation", "Skip expensive validation of Dawn commands."}}}};
+              {"skip_validation", "Skip expensive validation of Dawn commands."}},
+             {Toggle::UseSpvc,
+              {"use_spvc",
+               "Enable use of spvc for shader compilation, instead of accessing spirv_cross "
+               "directly."}}}};
     }  // anonymous namespace
 
     void TogglesSet::SetToggle(Toggle toggle, bool enabled) {

src/dawn_native/Toggles.h

@@ -33,7 +33,7 @@ namespace dawn_native {
         UseD3D12ResourceHeapTier2,
         UseD3D12RenderPass,
         SkipValidation,
-
+        UseSpvc,
         EnumCount,
         InvalidEnum = EnumCount,
     };