Revert "Use libshaderc_spvc as a source_set, so complete_static_lib applies properly"

This reverts commit 97c3be2699.

Reason for revert: Breaks linking on MSVC

Original change's description:
> Use libshaderc_spvc as a source_set, so complete_static_lib applies properly
> 
> Brings the list of dependencies for a standalone Dawn app down from:
> 
>   obj/libdawn_native.a
>   obj/libdawn_utils.a
>   obj/src/dawn/libdawn_proc.a
>   obj/third_party/shaderc/libshaderc_spvc/spvc.o
>   obj/third_party/shaderc/libshaderc_spvc/spvc_private.o
>   obj/third_party/shaderc/libshaderc_spvc/spvcir_pass.o
>   obj/third_party/SPIRV-Tools/libspvtools_opt.a
> 
> to
> 
>   obj/libdawn_native.a
>   obj/src/dawn/libdawn_proc.a
> 
> Bug: dawn:327
> Change-Id: I74654b304a9cb5f2aff19e72aa6a8bf1eb708c15
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/15481
> Reviewed-by: Kai Ninomiya <kainino@chromium.org>
> Reviewed-by: Ryan Harrison <rharrison@chromium.org>
> Commit-Queue: Kai Ninomiya <kainino@chromium.org>

TBR=kainino@chromium.org,rharrison@chromium.org

Change-Id: Ib6d17eccebf371b71e74f1857b50bb3cfd67595a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: dawn:327
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/15722
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>

BUILD.gn

@@ -158,7 +158,7 @@ source_set("libdawn_native_sources") {
   # libdawn_native target
   public_deps = [
     ":dawn_platform",
-    "${dawn_shaderc_dir}:libshaderc_spvc_sources",
+    "${dawn_shaderc_dir}:libshaderc_spvc",
   ]
 
   sources = get_target_outputs(":libdawn_native_utils_gen")