[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