[libc++][P1115][C++20] Improving the Return Value of Erase-Like Algorithms II: Free...

[libc++][P1115][C++20] Improving the Return Value of Erase-Like Algorithms II: Free erase/erase if.

Summary:
This patch adds return type to std::erase and std::erase_if functions.

Also:
* Update __cpp_lib_erase_if to 202002L.
* Fix synopsis in unordered_map.
* Fix generate_feature_test_macro_components.py script.

Reviewers: EricWF, mclow.lists, ldionne, #libc

Reviewed By: ldionne, #libc

Subscribers: broadwaylamb, zoecarver, dexonsmith, ldionne, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D75905