Add support for multisampled textures.

This Cl adds the AST, WGSL-Reader and SPIRV-Writer support for the
`texture_multisampled_2d` type.

Bug: tint:237
Change-Id: Ib3eb831227b49f32162801f1e1b4e474774c78b3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/28480
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: David Neto <dneto@google.com>

src/reader/wgsl/parser_impl.h

diff --git a/src/reader/wgsl/parser_impl.h b/src/reader/wgsl/parser_impl.h
index eb35b3d..6da84df 100644
--- a/src/reader/wgsl/parser_impl.h
+++ b/src/reader/wgsl/parser_impl.h
@@ -188,6 +188,10 @@
   /// Parses a `sampler_type` grammar element
   /// @returns the parsed Type or nullptr if none matched.
   ast::type::Type* sampler_type();
+  /// Parses a `multisampled_texture_type` grammar element
+  /// @returns returns the multisample texture dimension or kNone if none
+  /// matched.
+  ast::type::TextureDimension multisampled_texture_type();
   /// Parses a `sampled_texture_type` grammar element
   /// @returns returns the sample texture dimension or kNone if none matched.
   ast::type::TextureDimension sampled_texture_type();