CMake: bump warning level from 3 to 4 for msvc builds

This enables, among other things, warnings on unused variables, which
matches our default Clang and GCC builds.

Change-Id: I3f405b1a5c30e428542cd72d50f07910e477cea6
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/44880
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 896b1d1..92669f9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -183,7 +183,7 @@
     target_compile_options(${TARGET} PRIVATE
       /bigobj
       /EHsc
-      /W3
+      /W4
       /WX
       /wd4068
       /wd4244