Revert "Remove support for # comments."

This reverts commit 6e570a9360.

Reason for revert: Missed Dawn tests

Original change's description:
> Remove support for # comments.
>
> This CL removes support for parsing # style comments. Only the //
> comments are accepted now.
>
> Change-Id: I062c2a86149ecf828723c74f7168243678ee5870
> Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/38823
> Commit-Queue: dan sinclair <dsinclair@chromium.org>
> Commit-Queue: Ben Clayton <bclayton@google.com>
> Auto-Submit: dan sinclair <dsinclair@chromium.org>
> Reviewed-by: Ben Clayton <bclayton@google.com>

TBR=dneto@google.com,dsinclair@chromium.org,bclayton@google.com

Change-Id: Iaf19ab9def92ad10c27d70480d259e25f3da719d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/39061
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>

src/reader/wgsl/lexer.cc

@@ -143,7 +143,8 @@ void Lexer::skip_whitespace() {
 }
 
 void Lexer::skip_comments() {
-  if (!matches(pos_, "//")) {
+  // TODO(bclayton): Remove support for # comments
+  if (!matches(pos_, "#") && !matches(pos_, "//")) {
     return;
   }