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
Also, this adds unit tests to check that limits.h complies with the C
standard.
Reviewed By: sivachandra
Differential Revision: https://reviews.llvm.org/D110643