We'll want to use this for ObjC as well, so we'll make it public and put
it in a public place.

This means that the two compilers will update together, and that ObjC
has the list behavior that C does.

…mpiler

We'll want to use this for the ObjC++ compiler too.

So we can re-use it for the ObjC code

So we can re-use it in the ObjC++ standards

To avoid duplication between C and ObjC

Check clang-cl as well as cl, and clang as well as gcc.

This tests ObjC and ObjC++ both with and without C enabled. I did this
because I ran into issues where ObjC only worked when C was enabled, and
then a later bug where C was disabled, due to the fact that C and ObjC
both use `c_std` and not `objc_std`.

Since this is optional, we should not accept that GCC is a valid ObjC or
G++ is a valid ObjC++ Compiler unless we've tested that they can
actually do a basic compile.

This requires fixing a number of tests that have broken assumptions. In
some cases I've split tests where issues with one language would hide
the other. It would be great if we had a competent test framework that
allowed subtests to skip, unfortunately we have python's unittest
instead. Because of that we can't avoid extra tests by use of subtests.