[clang] [clang] Support constexpr bitcasts between equal-size vector and integer types (PR #167016)

[Yanzuo Liu via cfe-commits]

zwuis wrote:

Thank you for your patch! Please [reference](https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue) the issue fixed by this PR in PR description.

> Please let me know if further modifications or additional test coverage are recommended.

Tests are required. You might refer to PRs solving sub-issues of #30794 for writing tests.

https://github.com/llvm/llvm-project/pull/167016