[libc][NFC] Use a function instead of templated static member functions for TYPE_DESC

I'm surprised that clang accepts the current code.
It seems odd to me to specialize templated static member variables.

GCC rejects them: https://godbolt.org/z/3ecE9Ps7T

This patch is in the context of https://github.com/llvm/llvm-project/issues/59368

Differential Revision: https://reviews.llvm.org/D140981