[clang driver] Move default module cache from system temporary directory

1) Shared writable directories like /tmp are a security problem.
2) Systems provide dedicated cache directories these days anyway.
3) This also refines LLVM's cache_directory() on Darwin platforms to use
   the Darwin per-user cache directory.

Reviewers: compnerd, aprantl, jakehehrlich, espindola, respindola, ilya-biryukov, pcc, sammccall

Reviewed By: compnerd, sammccall

Subscribers: hiraditya, llvm-commits, cfe-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D82362