Modify FunctionImport to take a callback to load modules

When linking static archive, there is no individual module files to
load. Instead they can be mmap'ed and could be initialized from a
buffer directly. The callback provide flexibility to override the
scheme for loading module from the summary.

Differential Revision: http://reviews.llvm.org/D15101

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 254479