[ESIMD][NFC] Extract ESIMD handling from sycl-post-link to library.

[maksimsab]

This change extracts handleESIMD from sycl-post-link to SYCLPostLink component for reuse in NewOffloading compilation flow in clang and for reuse in sycl-jit.

The body of handleESIMD function is refactored for better readability.

The documentation of lowerESIMDConstructs is fixed and the argument name is changed to more straightforward.

[maksimsab]

Speaking of the testing, different parts are being tested by LIT tests by sycl-post-link. Linking is tested independently from sycl. Lowering is tested by LITs. The control flow of handleESIMD is difficult to test by unittests.

[asudarsa]

Hi @maksimsab I had a look at the build fails. Not really clear what the issue is. Did you figure it out? Thanks

[maksimsab]

@asudarsa CI issues are really not related to this PR, I can see similar issues in https://github.com/intel/llvm/pull/19022 .

@intel/llvm-gatekeepers Can we merge this?

[dm-vodopyanov]

@asudarsa CI issues are really not related to this PR, I can see similar issues in #19022 .

@intel/llvm-gatekeepers Can we merge this?

Could you please create issues against these failures?

[github-actions[bot]]

@intel/llvm-gatekeepers please consider merging

[sarnex]

@maksimsab is this actually ready for merge?

[github-actions[bot]]

@intel/llvm-gatekeepers please consider merging

[sarnex]

@maksimsab x2 is this actually ready for merge?

[maksimsab]

@sarnex thanks for pointing out!

@intel/llvm-gatekeepers Can we merge this please?