Extend RemapInstruction and friends to take an optional new parameter, a ValueMaterializer.

Extend LinkModules to pass a ValueMaterializer to RemapInstruction and friends to lazily create Functions for lazily linked globals. This is a big win when linking small modules with large (mostly unused) library modules.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182776 91177308-0d34-0410-b5e6-96231b3b80d8