Captured vars

This pull request includes
* support for variables captured in closures*,
* a fix for issue #8512: arguments of non-immediate type (structs, tuples, etc) passed by value can now be accessed correctly in GDB. (I managed to fix this by using `llvm::DIBuilder::createComplexVariable()`. However, I am not sure if this relies on unstable implementation details of LLVM's debug info handling. I'll try to clarify this on the LLVM mailing list).
* simplification of the `debuginfo` module's public interface: the caller of functions like `create_local_var_metadata()` doesn't have to know and catch all cases when it mustn't call the function,
* a cleanup refactoring with unified handling for locals, [self] arguments, captured variables, and match bindings,
* and proper span information for self arguments.

\* However, see comment at https://github.com/michaelwoerister/rust/blob/1d916ace136a27e354d73d65f488603c65f65bd2/src/test/debug-info/var-captured-in-nested-closure.rs#L62 . This is the same problem as with the fix for issue #8512 above: We are probably using `llvm.dbg.declare` in an unsupported way that works today but might not work after the next LLVM update.

Cheers,
Michael

Fixes #8512
Fixes #1341