[c++] implements tentative DR1432 for partial ordering of function template

[c++] implements tentative DR1432 for partial ordering of function template

D128745 handled DR1432 for the partial ordering of partial specializations, but
missed the handling for the partial ordering of function templates. This patch
implements the latter. While at it, also simplifies the previous implementation to
be more close to the wording without functional changes.

Fixes https://github.com/llvm/llvm-project/issues/56090

Reviewed By: erichkeane, #clang-language-wg, mizvekov

Differential Revision: https://reviews.llvm.org/D133683