[libcxx] Introduce LinuxRemoteTI for remote testing

Summary:
This patch adds a new target info object called LinuxRemoteTI.
Unlike LinuxLocalTI, which asks the host system about various things
like available locales, distribution name etc. which don't make sense
if we're testing on a remote board, LinuxRemoteTI uses SSHExecutor
to get information from the target system.

Reviewers: jroelofs, ldionne, bcraig, EricWF, danalbert, mclow.lists

Reviewed By: jroelofs

Subscribers: christof, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D72847