[ThinLTO] Leverage new in-place renaming support

Due to the new in-place renaming support added in r257174, we no
longer need to invoke ThinLTO global renaming from clang. It will be
invoked on the module in the FunctionImport pass (by an immediately
following llvm commit).

As a result, we don't need to load the FunctionInfoIndex as early,
so that is moved down into EmitAssemblyHelper::EmitAssembly.

llvm-svn: 257179