Reland "Remove support for # comments."

This reverts commit 21b52b60b6.

Reason for revert: Dawn fixes landed, trying again.

Original change's description:
> 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>

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

# Not skipping CQ checks because this is a reland.

Change-Id: I91a478237bc9853108e1ce97b9fdc2ce01841998
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/39221
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>

src/reader/wgsl/lexer.cc

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