Resume inlining globals across crates

[alexcrichton]

In #8185 cross-crate condition handlers were fixed by ensuring that globals
didn't start appearing in different crates with different addressed. An
unfortunate side effect of that pull request is that constants weren't inlined
across crates (uint::bits is unknown to everything but libstd).

This commit fixes this inlining by using the available_eternally linkage
provided by LLVM. It partially reverts #8185, and then adds support for this
linkage type. The main caveat is that not all statics could be inlined into
other crates. Before this patch, all statics were considered "inlineable items",
but an unfortunate side effect of how we deal with &static and &[static]
means that these two cases cannot be inlined across crates. The translation of
constants was modified to propogate this condition of whether a constant
should be considered inlineable into other crates.

Closes #9036

[alexcrichton]

I believe that this, plus some minor enhancements about the address_insignificant attribute, are all that's left blocking using format! by default. The story of inner statics has improved greatly over the past few weeks, and they should be usable in just about all situations now.

The last sticky point is that address_insignificant is causing linkage failures, but I believe that I have a good workaround for this, but it relies on many changes made in this pull request. Soon though, soon format! will be usable everywhere!