We see several occurrence of "diagnostic push/pop" so why not
make them macros.  Tested on GCC8 / Clang 6.

Note that ruby.h is intentionally left untouched because we don't
want to introduce new public macros.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64118 b2dd03c8-39d4-4d8f-98ff-823fe69b080e