Postgres & MySQL: cleanup AliasExpressionSegment

[jpy-git]

Brief summary of the change made

Makes progress on #1488.

The ANSI AliasExpressionSegment was updated in this commit to allow for some Exasol parsing (quoted literals for aliasing also applies to tsql and bigquery).
Postgres and MySQL are pretty basic in this regard and this ANSI segment is causing parsing issues in the linked issue so we can just overwrite them.

Are there any other side effects of this change that we should be aware of?

No

Pull Request checklist

• Please confirm you have completed any of the necessary steps below.

• Included test cases to demonstrate any code changes, which may be one or more of the following:

  • .yml rule test cases in test/fixtures/rules/std_rule_cases.
  • .sql/.yml parser test cases in test/fixtures/dialects (note YML files can be auto generated with tox -e generate-fixture-yml).
  • Full autofix test cases in test/fixtures/linter/autofix.
  • Other.

• Added appropriate documentation for the change.

• Created GitHub issues for any relevant followup/future enhancements if appropriate.

[codecov]

Codecov Report

Merging #2353 (409994e) into main (a50ccd8) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #2353   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          160       160           
  Lines        11383     11391    +8     
=========================================
+ Hits         11383     11391    +8     

Impacted Files	Coverage Δ
src/sqlfluff/dialects/dialect_mysql.py	100.00% <100.00%> (ø)
src/sqlfluff/dialects/dialect_postgres.py	100.00% <100.00%> (ø)

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