Debug support

[richardmembarth]

• add location information to nodes in thorin
• emit basic debug information (currently line info only)
• set -g flag by CMake

[leissa]

For the most part, this patch looks pretty straight-forward. I just have three remarks:

• Where is the loc actually glued into the llvm-IR?
• Maybe we should add a default location as default argument in World.
• Is there anything I missed which is open to debate or more complicated?

[richardmembarth]

• The loc is set here. This is done for primops only atm.
• There is nothing more complicated, we do not emit debug information for variables atm, this is still missing.

[leissa]

ah ok, missed that line.

[leissa]

Then we should merge.

[richardmembarth]

Removed assert from DefNode since we use the Location() default constructor now.