[Driver] Fix `ToolChain::getCompilerRTPath()` to return the correct path on Apple platforms.

When the target triple was an Apple platform `ToolChain::getOSLibName()`
(called by `getCompilerRTPath()`) would return the full OS name
including the version number (e.g. `darwin20.3.0`). This is not correct
because the library directory for all Apple platforms is `darwin`.

This in turn caused

* `-print-runtime-dir` to return a non-existant path.
* `-print-file-name=<any compiler-rt library>` to return the filename
  instead of the full path to the library.

Two regression tests are included.

rdar://77417317

Differential Revision: https://reviews.llvm.org/D101682