[driver] Refactor getRuntimePaths. NFC

This used to be getRuntimePath till https://reviews.llvm.org/D115049
added a fallback search path for Android. As far as I can tell, the
intent has always been to use the first existing path though instead of
actually supporting multiple runtime paths. We can move the existence
checks into getRuntimePath and have it return std::optional, which also
makes the `--print-runtime-dir` behavior much cleaner.

The motivation is a follow-up change to Android runtime path searches,
which is much nicer with this in place.

Reviewed By: phosek, MaskRay

Differential Revision: https://reviews.llvm.org/D158475