Investigate sharing code with GCC/C++

Originally posted by @CohenArthur in https://github.com/Rust-GCC/gccrs/pull/1329#pullrequestreview-1014858287:

[...], we might even think about simply linking against the C++ const folder in the future, [...]

Originally posted by @dafaust in https://github.com/Rust-GCC/gccrs/pull/1329#pullrequestreview-1015852117:

This is an interesting idea, I'd be curious to see if/how it works in practice. I was actually wondering whether some of the ported-but-not-renamed functions in this PR will give link errors if we try to build a gcc with both the rust and c++ frontends enabled. Definitely not a blocker for this to be merged!

It might turn out that if we want to link against the C++ const folder directly we'd need to factor it out of the C++ frontend to somewhere common, like how C and C++ share some functionality in the c-family directory. I'm not very familiar with the build processes for the front ends though so maybe these are issues long since solved :P I need to read up.

Start with the discussion in pull request #1329 and inspect the C++ const folder and the c-family directory mentioned there. Build GCC with both the Rust and C++ frontends enabled, checking whether the ported functions cause link errors. Done means documenting whether sharing is practical and identifying any common factoring required.