tint: Add _tint_materialize internal builtin

Returns a materialization of the given argument.

Bug: tint:1697
Change-Id: Id25f7e10baa884047af21f89245884c551560f7b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/104822
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/tint/sem/builtin_type.cc

@@ -360,6 +360,9 @@ BuiltinType ParseBuiltinType(const std::string& name) {
     if (name == "atomicCompareExchangeWeak") {
         return BuiltinType::kAtomicCompareExchangeWeak;
     }
+    if (name == "_tint_materialize") {
+        return BuiltinType::kTintMaterialize;
+    }
     return BuiltinType::kNone;
 }
 
@@ -589,6 +592,8 @@ const char* str(BuiltinType i) {
             return "atomicExchange";
         case BuiltinType::kAtomicCompareExchangeWeak:
             return "atomicCompareExchangeWeak";
+        case BuiltinType::kTintMaterialize:
+            return "_tint_materialize";
     }
     return "<unknown>";
 }

src/tint/sem/builtin_type.h

@@ -142,6 +142,7 @@ enum class BuiltinType {
     kAtomicXor,
     kAtomicExchange,
     kAtomicCompareExchangeWeak,
+    kTintMaterialize,
 };
 
 /// Matches the BuiltinType by name