Modernize C++ in runtime code

[steven-johnson]

We used to constrain our runtime code to pre-C++11, since we compiled it with an older standard to avoid threadsafe-static initialization.

As of #5303, we now use -fno-threadsafe-statics instead, so we can use (at least) C++11 features in runtime code now (e.g. nullptr).

Probably better to just wait until #5282 lands and upgrade the runtime code to C++17 where it makes sense to do so.