Refactor code to have rudimentary support for using SPVC

This is the first step for having a fully operational SPVC usage
path. This version of SPVC integration uses SPVC for setting up the
options to the compiler, but a lot of the actual interaction with
spirv-cross is done in Dawn, just via SPVC's compiler object.

Future CLs will migrate more of the spirv-cross interaction to using
the SPVC API, eventually removing the need for Dawn to know about
spirv-cross.

BUG=dawn:288

Change-Id: I68e0773f910d7fe967235b6987b3debe1d13883f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/14143
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/fuzzers/DawnSPIRVCrossGLSLFastFuzzer.cpp

@@ -35,7 +35,7 @@ namespace {
             options.SetGLSLLanguageVersion(440);
             options.SetFixupClipspace(true);
             if (context.InitializeForGlsl(input.data(), input.size(), options) ==
-                shaderc_compilation_status_success) {
+                shaderc_spvc_status_success) {
                 context.CompileShader(&result);
             }
         });

src/fuzzers/DawnSPIRVCrossHLSLFastFuzzer.cpp

@@ -41,7 +41,7 @@ namespace {
             options.SetHLSLPointCoordCompat(true);
             options.SetHLSLPointSizeCompat(true);
             if (context.InitializeForHlsl(input.data(), input.size(), options) ==
-                shaderc_compilation_status_success) {
+                shaderc_spvc_status_success) {
                 context.CompileShader(&result);
             }
         });

src/fuzzers/DawnSPIRVCrossMSLFastFuzzer.cpp

@@ -34,7 +34,7 @@ namespace {
 
             // Using the options that are used by Dawn, they appear in ShaderModuleMTL.mm
             if (context.InitializeForMsl(input.data(), input.size(), options) ==
-                shaderc_compilation_status_success) {
+                shaderc_spvc_status_success) {
                 context.CompileShader(&result);
             }
         });