TransformUtils: Introduce module splitter.

The module splitter splits a module into linkable partitions. It will
be used to implement parallel LTO code generation.

This initial version of the splitter does not attempt to deal with the
somewhat subtle symbol visibility issues around module splitting. These
will be dealt with in a future change.

Differential Revision: http://reviews.llvm.org/D12132

llvm-svn: 245662