[Bug]: Generated ISEnum instead of IS when using "is" as an enum key.

[xiangyan99]

Describe the bug

When "is" is used as an enum key in the spec, the generated key should be "IS_ENUM"; however, the actual value is "ISEnum".

Reproduction

Generated code using https://github.com/Azure/azure-rest-api-specs/tree/search_enum_issue_repro

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.

[msyyc]

Python Mgmt SDK also meets similar breaking: https://github.com/Azure/azure-sdk-for-python/pull/45080/changes#diff-1c5b3718da58997c37e3f5b20708da64230c2c6c9748b6065496166e09751569.

I remember we indeed add _ENUM instead of Enum so this may be roll back. Here is spector case PR #9785 for similar scenario.