Moved the builtins documentation to lib/builtins/

And fixed typos in the ASan readme.

Differential Revision: http://reviews.llvm.org/D3927

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@209778 91177308-0d34-0410-b5e6-96231b3b80d8