[Don't merge yet] Internalize everything but _Dmain when emitting a -singleobj executable.

This is just a first draft; there are certainly cases
where this still breaks (e.g. naked inline asm).

The general direction looks promising, though: Building a
fully optimized Phobos "Hello World" is noticably faster
with the internalizing enabled (0.7s vs 1s) on my machine.

Enabling the pass by default on -O4 and above is more of
a cute idea (to finally do the flag description justice)
than a serious proposal.

Don't merge yet, this is just up for discussion.