[libclc] [libclc] Enable -fdiscard-value-names build flag to reduce bitcode size (PR #149016)

[Wenju He via cfe-commits]

wenju-he wrote:

> I think this could optionally do with a comment before explaining what this flag is helping to achieve, but it's not a blocker.

it is explained at https://clang.llvm.org/docs/UsersManual.html#id72 and reducing size and verbosity is what this flag is intended for. I mean the flag doesn't have another meaning.

https://github.com/llvm/llvm-project/pull/149016