[ThinLTO] Use new in-place symbol changes for exporting module

Due to the new in-place ThinLTO symbol handling support added in
r257174, we now invoke renameModuleForThinLTO on the current
module from within the FunctionImport pass.

Additionally, renameModuleForThinLTO no longer needs to return the
Module as it is performing the renaming in place on the one provided.

This commit will be immediately preceeded by a companion clang patch to
remove its invocation of renameModuleForThinLTO.

llvm-svn: 257181