Fixed PlatformDarwin to locate Xcode by using HostInfo::GetProgramFileSpec()....

Fixed PlatformDarwin to locate Xcode by using HostInfo::GetProgramFileSpec(). This function returns the FileSpec to the program that is running the LLDB.framework or lldb.so and is more reliable than checking the path of LLDB.framework/lldb.so itself since it might not exist within the Xcode.app bundle (DYLD_FRAMEWORK_PATH). Then we check DEVELOPER_DIR, then check the currently installed Xcode with xcrun.

<rdar://problem/23167253>

llvm-svn: 250772