Implement an address_insignificant attribute

[alexcrichton]

This can be applied to statics and it will indicate that LLVM will attempt to
merge the constant in .data with other statics.

I have preliminarily applied this to all of the statics generated by the new
ifmt! syntax extension. I compiled a file with 1000 calls to ifmt! and a
separate file with 1000 calls to fmt! to compare the sizes, and the results
were:

fmt           310k
ifmt (before) 529k
ifmt (after)  202k

This now means that ifmt! is both faster and smaller than fmt!, yay!

[sanxiyn]

In the long term, shouldn't address_insignificant be the default, and address_significant marked by the attribute? Rust statics are constant...

[alexcrichton]

I found out that llvm won't optimize this unless the constant also has an internal-type linkage. Thatmeans that statics marked pub won't benefit from this.

Additionally though, TLS keys all have significant addresses, and in my opinion it would be a real bummer to have to mark all of our tls keys with address_significant. An awesome solution to that possibly would be exportable macros from tls (and the macro throws on address_significant).

For now I think that normal statics shouldn't have this thrown on by default for the TLS reason, but other than that use case it's certainly worth a shot to see what starts crashing and burning.