Fix a problem with debug info for nested local variables.

[redstar]

Debug info for nested local variables must be created for each
function (with different address expression). The resulting
debug info was not stored per function which resulted in the
use of the last created debug info even if it was wrong.

This commit fixes the bug and the current assertion with LLVM 3.7.

[dnadlinger]

To me, it'd seem more natural to store a map from declarations to DIVariables in IrFunction, but I guess the two approaches should be roughly equivalent as long as the code is not really hot. I'd then also use a llvm::DenseMap or something like that, but I'm probably taking my dislike for std::map a bit too far. ;)

[redstar]

Yes, I also think that IrVar is not the best choice for the mapping. But it was the simple way to fix the bug. I try a refactoring soon. Some kind of hashmap would be better than std::map. I'll have a look at llvm::DenseMap.