[clang] [clang][driver] Use platform specific calls to get the executable absolute path (PR #68091)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

FWIW, I'd be quite wary about modifying the logic here (whatever it is - I haven't checked closely); if we used to resolve to one point of a symlink before, and now resolve to the other end, I can almost guarantee you that there are dozens of existing users that rely on the current behaviour - that would be broken if it is changed.

https://github.com/llvm/llvm-project/pull/68091