[Parse] Move standalone_dollar_identifier diagnosis to Parser.

[mininny]

The compiler can't make up its mind as to whether $ is an identifier:

$ echo 'operator $' | swiftc -
<stdin>:1:10: error: '$' is not an identifier; use backticks to escape it
operator $
         ^
         `$`
<stdin>:1:10: error: '$' is considered an identifier and must not appear within an operator name
operator $
         ^

The first error should probably not be emitted in this context.

This PR moves dollar identifier diagnosis to Parser, and fixes the diagnosis for identifier_within_operator_name.

This is my first time committing to Swift repository, so if there are any issues, I'd appreciate any feedbacks! :)

Resolves SR-13092.

[rintaro]

Thank you for the contribution! I think this is a good idea.

[mininny]

Thank you @rintaro! I resolved all your comments.

[varungandhi-apple]

Very small request: could you add "Resolves SR-13092." to the commit message and "// SR-13092" next to the test case you added? That would be helpful information in case anyone looks at the change in the future.

[mininny]

@varungandhi-apple Done! :)

[varungandhi-apple]

@swift-ci please smoke test

[rintaro]

@swift-ci Please smoke test Linux

[mininny]

Fixed the comment! :) I'm not sure why the Linux test wasn't passing..

[xwu]

@swift-ci please smoke test

[AnthonyLatsis]

Just one optional suggestion

[rintaro]

@swift-ci Please smoke test

[rintaro]

Thank you Minhyuk!