[lldb] Remove undocumented return value from DiagnosticManager::PutString

The returned value is currently unused. It also seems to imply that
it somehow represents 'printf-style' the number of characters/bytes
written to some output stream (which is incorrect, as we only know
the actual size of the written message when we have rendered it,
e.g. via GetString and DiagnosticManagers have no associated
output stream).

llvm-svn: 368577