Implement P1771

Implement P1771

As passed in the Cologne meeting and treated by Core as a DR,
[[nodiscard]] was applied to constructors so that they can be diagnosed
in cases where the user forgets a variable name for a type.

The intent is to enable the library to start using this on the
constructors of scope_guard/lock_guard.

Differential Revision: https://reviews.llvm.org/D64914

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@367027 91177308-0d34-0410-b5e6-96231b3b80d8