Update PlatformDarwin::GetDeveloperDir to handle the two

most common cases where the Xcode.app bundle puts lldb -
either as a default part of the bundle, or in a toolchain
subdirectory, so the platform subclasses can find files
relative to this directory.

Dropped support for handling the case where the lldb
framework was in /Library/PrivateFrameworks.  I think
this was intended to handle the case where lldb is installed
in / (outside the Xcode.app bundle) - but in that case, we
can look in the raw directory file paths to find anything.

<rdar://problem/35285622> 

llvm-svn: 320240