Replace cp -a in various Clang tests

Summary:
cp -a is neither part of POSIX nor the LSB. The nearest equivalent under
POSIX is cp -RPp; however, cp -R is sufficient for the intended purpose.

test/Modules/crash-vfs-headermaps.m is not updated since it requires
system-darwin anyway.

Reviewers: bruno

Reviewed By: bruno

Subscribers: bruno, rcraik, cfe-commits

Differential Revision: https://reviews.llvm.org/D41545

llvm-svn: 321778