[libc][NFC] Move utils/CPP to src/__support/CPP.

The idea is to move all pieces related to the actual libc sources to the
"src" directory. This allows downstream users to ship and build just the
"src" directory.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D112653

GitOrigin-RevId: f362aea42d29da2c8620c6eb8902f31d01613edc
65 files changed