[ir][msl] Fix const of non-entrypoint pointers

Add the `const` modifier to the non-entrypoint pointer parameters

Bug: tint:1967
Change-Id: I0e3ce118544ae257704fb696056a43f3ab927a62
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/162822
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Auto-Submit: dan sinclair <dsinclair@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
diff --git a/test/tint/bug/chromium/1433499.wgsl.expected.ir.msl b/test/tint/bug/chromium/1433499.wgsl.expected.ir.msl
index 2c9000e..95b1b95 100644
--- a/test/tint/bug/chromium/1433499.wgsl.expected.ir.msl
+++ b/test/tint/bug/chromium/1433499.wgsl.expected.ir.msl
@@ -1,6 +1,6 @@
 #include <metal_stdlib>
 using namespace metal;
 
-void f(thread float* p) {
+void f(thread float* const p) {
   thread float* const x = p;
 }