llgoi: Fix type identity for imported binary packages.

go/loader creates a fresh package map for each source package it imports. In
llgoi this caused binary imported packages to be imported anew for every input
line, resulting in spurious type errors and panics in go/ssa when encountering
previously imported types. Fix this by setting types.Config.Packages to our
internal package map.

Differential Revision: http://reviews.llvm.org/D8409

llvm-svn: 232617