[Backend][HLS] AST for XLS DSLX Backend

[AnthonyBSong]

Description

(Brief description of what this PR is about, what kind of problem you want to solve, and what is your solution)

Problems

(What this PR is about)

Proposed Solutions

(What changes have you made)

Examples

(Please provide an example of the input program and the expected behavior, e.g., the generated IR.)

Checklist

Please make sure to review and check all of these items:

• PR's title starts with a category (e.g. [Bugfix], [IR], [Builder], etc)
• All changes have test coverage (It would be good to provide ~2 different test cases to test the robustness of your code)
• Pass the formatting check locally
• Code is well-documented