[libc] Split out src/__support/alloc-checker.h (#173104)

This moves the libc-internal AllocChecker API out of
src/__support/CPP/new.h and updates CPP/README.md to state the
intent to keep src/__support/CPP and the LIBC_NAMESPACE::cpp
namespace a "pure" subset of standard C++ API polyfills.

GitOrigin-RevId: 2e16cadd560f760f100030e575fe402f3f6b2eba