Go bindings: make various DIBuilder arguments optional.

r234262 changed some code in DIBuilderBindings.cpp to use the unwrap function
to unwrap debug metadata. The problem with this is that unwrap asserts that
its argument is non-null, which is not what we want in a number of places
in DIBuilder where the argument is optional. This change makes certain
arguments optional by adding null checks in places where it is required,
fixing the llgo build.

llvm-svn: 234428