[libc++][math] Mathematical special functions: Implementing std::assoc_legendre

[PaulXiCao]

Implementing the associated Legendre polynomials std::assoc_legendre, std::assoc_legendref, std::assoc_legendrel which are part of C++17's mathematical special functions.

I started out from this abandoned merge request: https://reviews.llvm.org/D58876.

Documentation in libcxx/docs/Status/ is not yet included. This will be added after the integration of #89982.

The implementation dispatches to std::legendre which is waiting for integration in #92175.

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