[driver][macOS] Link libarclite from the default toolchain when clang

is running in a toolchain outside of xcode

'libarclite' usually lives in the same toolchain as 'clang'. However, the
Swift open source toolchains for macOS distribute Clang without 'libarclite'.
In that case, to allow the linker to find 'libarclite', we point to the
'libarclite' that should be in the XcodeDefault toolchain instead. The
path to the toolchain is inferred from the SDK path if it's specified.

https://bugs.swift.org/browse/SR-9972
rdar://49947573

llvm-svn: 359353