add ABIInfo trait and separate x86-64 ABI implementation from foreign trans

[crabtw]

ABIInfo provides a method to translate the type of foreign function.
The foreign trans uses the method to get ABI-specific type and uses the result to generate LLVM instructions for wrapper and shim functions.

[catamorphism]

Sorry, but can you rebase this against the latest incoming and re-submit? foreign has already been changed, and I'm afraid of losing something if I try to do the merge.

[brson]

Very cool. I'm glad to see this refactoring.

[crabtw]

I rebased and pushed.

[catamorphism]

Thanks!

[catamorphism]

Sorry, I had to revert this because it broke the Windows build. One of us will un-revert once we figure out what's going on.

[graydon]

Thanks for the quick backout. Sorry for the abrupt comment -- was on a cell phone. Just trying to get an all-green to snapshot.

[catamorphism]

No worries, a snapshot will be amazing!

[catamorphism]

@crabtw - Do you have access to a Windows machine? This patch is still failing on Windows: http://buildbot.rust-lang.org/builders/try-win/builds/210/steps/compile/logs/stdio -- and I don't really know where to start as to figuring out why.

[crabtw]

@catamorphism Sorry, I have no Windows machine. https://github.com/mozilla/rust/pull/4590/files#L0R166 is the problem. Replacing the condition with llvm::LLVMGetTypeKind(self.ret_ty.ty) == Void should work.

[catamorphism]

@crabtw Ok, thanks -- I tried making that change and am running try.

[catamorphism]

@crabtw Thanks, that more or less did the trick. I landed this for real in 41adf9d