[Driver] Search computed sysroot for libc++ header paths

Summary:
The Android NDK's clang driver is used with an Android -target setting,
and the driver automatically finds the Android sysroot at a path
relative to the driver. The sysroot has the libc++ headers in it.

Remove Hurd::computeSysRoot as it is equivalent to the new
ToolChain::computeSysRoot method.

Fixes PR46213.

Reviewers: srhines, danalbert, #libc, kristina

Reviewed By: srhines, danalbert

Subscribers: ldionne, sthibaul, asb, rbar, johnrusso, simoncook, sabuasal, niosHD, jrtc27, MaskRay, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, PkmX, jocewei, Jim, lenary, s.egerton, pzheng, sameer.abuasal, apazos, luismarques, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81622