[Driver] Obey computed sysroot when finding libc++ headers.

Summary:
A handful of targets will try some default paths if --sysroot is not provided.
If that is the case, it should be used for the libc++ header paths.

Reviewers: srhines, EricWF

Reviewed By: srhines

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D45292

llvm-svn: 331389