[DB2] Fix parsing of string identifiers

[borchero]

Brief summary of the change made

Fixes #4133.

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

n/a

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

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (6c21775) compared to base (5364c7e).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #4134   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          195       195           
  Lines        14922     14922           
=========================================
  Hits         14922     14922           

Impacted Files	Coverage Δ
src/sqlfluff/dialects/dialect_db2.py	100.00% <100.00%> (ø)

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