[lldb/PlatformDarwin] Be more robust in computing the SDK path with xcrun

The current implementation isn't very resilient when it comes to the
output of xcrun. Currently it cannot deal with:

 - Trailing newlines.
 - Leading newlines and errors/warnings before the Xcode path.
 - Xcode not being named Xcode.app.

This extract the logic into a helper in PlatformDarwin and fixes those
issues. It's also the first step towards removing code duplication
between the different platforms and downstream Swift.

Differential revision: https://reviews.llvm.org/D76261