[Diagnostics] Put "deprecated copy" warnings into -Wdeprecated-copy

Summary:
GCC 9 added -Wdeprecated-copy (as part of -Wextra). This diagnostic is already implemented in Clang too, just hidden under -Wdeprecated (not on by default).
This patch adds -Wdeprecated-copy and makes it compatible with GCC 9+.
This diagnostic is heavily tested in deprecated.cpp, so I added simple tests just to check we warn when new flag/-Wextra is enabled.

Reviewers: rsmith, dblaikie

Reviewed By: dblaikie

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70342