[libc] Lay out framework for fuzzing libc functions.
Added fuzzing test for strcpy and some documentation related to fuzzing.
This will be the first step in integrating this with oss-fuzz.
Reviewers: sivachandra, abrachet
Reviewed By: sivachandra, abrachet
Subscribers: gchatelet, abrachet, mgorny, MaskRay, tschuett, libc-commits
Differential Revision: https://reviews.llvm.org/D74091