[llvm] Enable logf128 constant folding for hosts with 128bit long double (PR #96287)

[Nikita Popov via llvm-commits]

nikic wrote:

Reverted to fix the buildbot failures.

Before reapplying this, please also make sure that you fix the 1.5% regression to clang build time this causes. Presumably this is due to including `<cmath>` in a core header. Probably most easily tested using the DivisionByConstantInfo.cpp file.

https://github.com/llvm/llvm-project/pull/96287