Add few docs and implementation of strcpy and strcat.

Add few docs and implementation of strcpy and strcat.

Summary:
This patch illustrates some of the features like modularity we want
in the new libc. Few other ideas like different kinds of testing, redirectors
etc are not yet present.

Reviewers: dlj, hfinkel, theraven, jfb, alexshap, jdoerfert

Subscribers: mgorny, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67867

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373764 91177308-0d34-0410-b5e6-96231b3b80d8