[clang] [llvm] [TLI] Add support for the `ilogb` libcall. (PR #112725)

[via cfe-commits]

c8ef wrote:

```
PASS: Clang Tools :: clang-move/move-used-helper-decls.cpp (98 of 1256)
```

This test appears to be flaky. I'll update this branch and re-trigger the CI.

https://github.com/llvm/llvm-project/pull/112725