Add libcxxabi option back for sanitizer use.

Summary:
A prior refactoring accidentally dropped the case for using libc++abi as
the out-of-tree C++ runtime library for sanitizers. This patch restores
that functionality, which is used by Android, which can't depend on the
full libc++ for these libraries.

Reviewers: phosek, EricWF

Reviewed By: phosek

Subscribers: meikeb, kongyi, chh, mgorny, delcypher, llvm-commits, #sanitizers, pirama

Differential Revision: https://reviews.llvm.org/D49157

llvm-svn: 336749