[lldb/PlatformMacOSX] Re-implement GetDeveloperDirectory

GetDeveloperDirectory returns a const char* which is NULL when we cannot
find the developer directory. This crashes in
PlatformDarwinKernel::CollectKextAndKernelDirectories because we're
unconditionally assigning it to a std::string. Coincidentally I just
refactored a bunch of code in PlatformMacOSX so instead of a ad-hoc fix
I've reimplemented the method based on GetXcodeContentsDirectory.

The change is mostly NFC. Obviously it fixes the crash, but it also
removes support for finding the Xcode directory through he legacy
$XCODE_SELECT_PREFIX_DIR/usr/share/xcode-select/xcode_dir_path.

Differential revision: https://reviews.llvm.org/D76938