msl: Add `matrix` to list of reserved keywords

This is in the metal namespace, which we import fully.

Change-Id: I986cdebbe1897ad0b433bc0260480dd8839f93e0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/62400
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/transform/renamer.cc

@@ -745,6 +745,7 @@ const char* kReservedKeywordsMSL[] = {
     "long3",
     "long4",
     "main",
+    "matrix",
     "metal",
     "mutable",
     "namespace",

src/transform/renamer_test.cc

@@ -1045,6 +1045,7 @@ INSTANTIATE_TEST_SUITE_P(
         "long3",
         "long4",
         "main",   // No functions called main
+        "matrix",
         "metal",  // The namespace
         "packed_bool2",
         "packed_bool3",