[tint][utils] PascalCase tint::diag functions

Fixes a longstanding annoyance.

Change-Id: I39a9ba09e4efc8ed9570fa496b6dfcd66e8c830b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/173040
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
diff --git a/src/tint/lang/spirv/writer/common/helper_test.h b/src/tint/lang/spirv/writer/common/helper_test.h
index 1c08ab9..553b173 100644
--- a/src/tint/lang/spirv/writer/common/helper_test.h
+++ b/src/tint/lang/spirv/writer/common/helper_test.h
@@ -117,13 +117,13 @@
     bool Generate(Options options = {}, bool zero_init_workgroup_memory = false) {
         auto raised = Raise(mod, options);
         if (raised != Success) {
-            err_ = raised.Failure().reason.str();
+            err_ = raised.Failure().reason.Str();
             return false;
         }
 
         auto spirv = PrintModule(mod, zero_init_workgroup_memory);
         if (spirv != Success) {
-            err_ = spirv.Failure().reason.str();
+            err_ = spirv.Failure().reason.Str();
             return false;
         }