tint: Rework sem::Constant to be variant-of-vector

Instead of vector-of-variant.
This:
• Makes it impossible to produce a mix of scalar variant types, which
  would make no sense.
• Reduces the size of a Constant, by removing the union-tag from each
  element.

Also clean up terminology. Rename 'Constant::Scalar' to
 'Constant::Element'. Scalars are well-defined in WGSL, and with the
introduction of abstract-numerics, this no longer makes sense.

Bug: tint:1504
Change-Id: I599aa97ad1ea798b7db8e512a5990ba75827faad
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/91304
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/tint/utils/compiler_macros.h

@@ -0,0 +1,39 @@
+// Copyright 2022 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef SRC_TINT_UTILS_COMPILER_MACROS_H_
+#define SRC_TINT_UTILS_COMPILER_MACROS_H_
+
+#define TINT_REQUIRE_SEMICOLON \
+    do {                       \
+    } while (false)
+
+#if defined(_MSC_VER)
+// clang-format off
+#define TINT_BEGIN_DISABLE_WARNING_UNREACHABLE_CODE() \
+    __pragma(warning(push))                           \
+    __pragma(warning(disable:4702))                   \
+    TINT_REQUIRE_SEMICOLON
+#define TINT_END_DISABLE_WARNING_UNREACHABLE_CODE()   \
+    __pragma(warning(pop))                            \
+    TINT_REQUIRE_SEMICOLON
+// clang-format on
+#else
+// clang-format off
+#define TINT_BEGIN_DISABLE_WARNING_UNREACHABLE_CODE() TINT_REQUIRE_SEMICOLON
+#define TINT_END_DISABLE_WARNING_UNREACHABLE_CODE() TINT_REQUIRE_SEMICOLON
+// clang-format on
+#endif  // defined(_MSC_VER)
+
+#endif  // SRC_TINT_UTILS_COMPILER_MACROS_H_