[libclc] [libclc] only check filename part of the source for avoiding duplication (PR #135710)

[Wenju He via cfe-commits]

wenju-he wrote:

> You could just have `cos_fp32.cl` and `cos_fp16.cl`, of course.

Yes. To enable overriding generic implementation of `cos.cl`, the names in target folder could be `cos.cl` and `cos_fp16.cl`

> I would like to further investigate weak linkage as another means of overriding specific builtins down to a finer granularity. That approach might warrant another method of file discovery.

It would bring unnecessary builds and may increase build time. Filename overriding should be better.

https://github.com/llvm/llvm-project/pull/135710