debug info fixes/refactor

This fixes (on my end) #3340, #3117, #2945, #2922, and #2762

A general refactor of debug info generation in order to fix issues and
increase stability.

What I believe is the root cause of a bunch of issues is that we use the
temporary metadata/forward declarations too much (/ hold onto them for
too long). It seems to cause problems with the reference counting inside
LLVM.

This PR reduces the use of these forward declarations to a minimum, it
creates it, fills in the fields, and resolves it, instead of waiting
until the end of generating code.

Some smaller issues I came across have also been solved.

src/llvm_backend.hpp

@@ -199,8 +199,6 @@ struct lbModule {
 	RecursiveMutex debug_values_mutex;
 	PtrMap<void *, LLVMMetadataRef> debug_values; 
 
-	Array<lbIncompleteDebugType> debug_incomplete_types;
-
 	StringMap<lbAddr> objc_classes;
 	StringMap<lbAddr> objc_selectors;