Fix msvc build and enable multiprocessor builds

Disable warning C4324: 'struct_name' : structure was padded due to
__declspec(align())

Multiprocessor builds are not enabled by default when using msbuild,
unlike for Ninja builds.

Bug: tint:665
Change-Id: I0e50c88ac91acd963c31c07cbbf2fae6b743e77d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/45621
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

CMakeLists.txt

@@ -84,6 +84,11 @@ if (MSVC)
     endif()
 endif()
 
+# Enable msbuild multiprocessor builds
+if (MSVC)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /MP")
+endif()
+
 if (${TINT_CHECK_CHROMIUM_STYLE})
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Xclang -add-plugin -Xclang find-bad-constructs")
 endif()
@@ -197,6 +202,7 @@ function(tint_default_compile_options TARGET)
       /wd4127
       /wd4244
       /wd4267
+      /wd4324
       /wd4458
       /wd4514
       /wd4571