Make back::link run the LLVM linter

[catamorphism]

Once there are llvm-c bindings for the LLVM linter, run it from back::link::write::run_passes.

[sanxiyn]

It seems LLVMAddVerifierPass is now present.

[catamorphism]

The Lint pass is separate from the Verifier pass -- http://llvm.org/docs/doxygen/html/Lint_8cpp_source.html -- but I don't if there are bindings now. (I got this out of a FIXME so I don't really know the whole deal.)

[pcwalton]

I don't believe this is backwards incompatible, renominating.

[graydon]

accepted for well-covered milestone

[thestinger]

@Aatch added a -Z lint-llvm flag for this - do we actually want it on by default?

[emberian]

Sounds like a bad idea to me, considering how much noise it makes (slash how bad codegen is)