Cleaup of requirements for optional. While researching LWG3196, I realized...

Cleaup of requirements for optional. While researching LWG3196, I realized that optional did not reject 'const in_place_t' like it should. Added a test as well, and a check for arrays (which were already disallowed, but now we get a better error message). Should not affect anyone's code.

llvm-svn: 356918