[lldb] Refactor Symbols::DownloadObjectAndSymbolFile

 - Reduce indentation
 - Extract caching of the DbgShellCommand and the dsymForUUID executable
   (or equivalent)
 - Check the DBGShellCommands before falling back to
   /usr/local/bin/dsymForUUID
 - Don't check ~rc/bin/dsymForUUID
 - Improve error reporting
 - Don't cache the value of LLDB_APPLE_DSYMFORUUID_EXECUTABLE

Differential revision: https://reviews.llvm.org/D131303