LLVM backend: generate DIGlobalVariable's for non-function globals and
rename linkage names when exporting functions and globals.

zig_llvm.cpp: add some wrappers to convert a handful of DI classes
into DINode's since DIGlobalVariable is not a DIScope like the others.

zig_llvm.cpp: add some wrappers to allow replacing the LinkageName of
DISubprogram and DIGlobalVariable.

zig_llvm.cpp: fix DI class mixup causing nonsense reinterpret_cast.

The end result is that GDB is now usable since you now no longer need
to manually cast every global nor fully qualify every export.