[llgo] drop debug/DIBuilder.Declare

Summary:
Companion to D11864, which updates the
DIBuilder bindings to match a recent
change in the underlying API.

We drop the debug/DIBuilder.Declare method
for now, which is unused, as otherwise it will
be broken with the new DIBuilder API in the
Go bindings.

Reviewers: pcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11891

llvm-svn: 246545