[Bug lto/89358] [7/8/9 Regression] Combining -std=c++14 and -std=c++17 objects gives ODR warnings

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89358

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That change introduces some differences between C++98 and later modes, but the
testcase use C++14 and C++17. There are no differences in the code. There's
certainly no difference between std::less<int> in the two modes.

Even so, we need to be able to make parts of the std::lib conditional on the
standard version, as long as the semantics remain the same. These warnings are
not helpful if they make users worry about harmless implementation details.