[llvm] Enable logf128 constant folding for hosts with 128bit long double (PR #104929)

[Matthew Devereau via llvm-commits]

MDevereau wrote:

@chapuni Would you be happy with this patch if I relanded it with constant folding enabled by default but with a way to opt out of the behaviour via CMake?

https://github.com/llvm/llvm-project/pull/104929