Add mechanism for gradually phasing in new byte compilation warnings

Add mechanism for gradually phasing in new byte compilation warnings

* lisp/Makefile.in (BYTE_COMPILE_FLAGS): Enable all byte
compilation warnings.
* lisp/emacs-lisp/bytecomp.el (byte-compile-warning-types): Add
docstrings-non-ascii-quotes and document new semantics for `all'
and t.
(byte-compile--emacs-build-warning-types): New constant.
(byte-compile-warning-enabled-p): Implement the new semantics.
(byte-compile-docstring-style-warn): Reinstate the Unicode quote
warning.