tint: Implement runtime quantizeToF16()

Fixed: tint:991
Fixed: tint:1741
Change-Id: I55dbabce6d28adf5abb710dc1e3e879c5aaa6be5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/108140
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/tint/sem/builtin_type.cc

@@ -210,6 +210,9 @@ BuiltinType ParseBuiltinType(const std::string& name) {
     if (name == "pow") {
         return BuiltinType::kPow;
     }
+    if (name == "quantizeToF16") {
+        return BuiltinType::kQuantizeToF16;
+    }
     if (name == "radians") {
         return BuiltinType::kRadians;
     }
@@ -492,6 +495,8 @@ const char* str(BuiltinType i) {
             return "pack4x8unorm";
         case BuiltinType::kPow:
             return "pow";
+        case BuiltinType::kQuantizeToF16:
+            return "quantizeToF16";
         case BuiltinType::kRadians:
             return "radians";
         case BuiltinType::kReflect:

src/tint/sem/builtin_type.h

@@ -92,6 +92,7 @@ enum class BuiltinType {
     kPack4X8Snorm,
     kPack4X8Unorm,
     kPow,
+    kQuantizeToF16,
     kRadians,
     kReflect,
     kRefract,