[llvm] [IR] Add llvm.sincospi intrinsic (PR #125873)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Currently, the default lowering of this intrinsic relies on the sincospi[f|l] functions being available in the target's runtime (e.g. libc).

Does this call actually exist anywhere? I do not see evidence of it existing in glibc, on windows, or macOS. macOS does appear to have __sincospi and __sincospi_stret. The current library info is reporting this phantom call as available by default 

https://github.com/llvm/llvm-project/pull/125873