First part of P0600 - '[[nodiscard] in the standard library'. Mark the...

First part of P0600 - '[[nodiscard] in the standard library'. Mark the 'empty()' methods of all the containers as nodiscard. If you're calling empty() w/o looking at the result, you probably meanto to call 'clear()'. c++2a only

llvm-svn: 318269