[Darwin toolchain] Tune the logic for finding arclite.

The heuristic used to determine where the arclite libraries are to be
found was based on the path of the `clang` executable. However, in some
scenarios the `clang` executable is within a toolchain that does not
have arclite. When this happens, derive the arclite paths from the
sysroot option.

This allows Clang to correctly derive the arclite directory in, e.g.,
Swift CI, using similar logic to what the Swift driver has been doing
for several years.

Patched by Doug Gregor.

Reviewed By: keith

Differential Revision: https://reviews.llvm.org/D130205

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