Like GlobalAddrInst, but for SILGlobalVariables. These would become the same instruction when SILGlobalVariable can replace AST-level global references.
Swift SVN r10510
This will let us control linkage and emit new variables independent of the AST in SILGen. In particular, for lazy global initialization, we need to emit a unique internal once predicate for every top-level pattern binding. Switching everything over is a bit much to reengineer all at once, so for now, it can coexist with the globals map that is already there.
Swift SVN r10509
