Add runtime toggle for using SPVC

This will be used in the future to choose between Dawn directly
accessing spirv-cross, and using spvc to mediate that interation.

This also adds in the spvc library as a dependency. This is what cause
the rollback last time, but that issue should be resolved.

BUG=dawn:281

Change-Id: Ic0b02d136ca05e2fa71844ebc85586b8866d5712
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/14122
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
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

@@ -33,6 +33,7 @@ namespace dawn_native {
         UseD3D12ResourceHeapTier2,
         UseD3D12RenderPass,
         SkipValidation,
+        UseSpvc,
 
         EnumCount,
         InvalidEnum = EnumCount,