[ast] Rename Builtin::kSampleId to Builtin::kSampleIndex

Other builtins use WGSL terms instead of SPIR-V terms too, and the
WGSL writer is relying on the output of `operator<<(Builtin)`, which
just stringifies the name of the enum. This also matches the
equivalent `semantic::Usage::kSampleIndex` enum.

Added test coverage for WGSL builtin generation.

Bug: tint:372
Change-Id: I8077d22c4a5ddf67b1ad07e7365453db74db8e7d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/41660
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Auto-Submit: James Price <jrprice@google.com>

src/ast/builtin.cc

@@ -59,8 +59,8 @@ std::ostream& operator<<(std::ostream& out, Builtin builtin) {
       out << "global_invocation_id";
       break;
     }
-    case Builtin::kSampleId: {
-      out << "sample_id";
+    case Builtin::kSampleIndex: {
+      out << "sample_index";
       break;
     }
     case Builtin::kSampleMaskIn: {

src/ast/builtin.h

@@ -32,7 +32,7 @@ enum class Builtin {
   kLocalInvocationId,
   kLocalInvocationIndex,
   kGlobalInvocationId,
-  kSampleId,
+  kSampleIndex,
   kSampleMaskIn,
   kSampleMaskOut,