[LTO] Add a function `LTOCodeGenerator::getMergedModule`

One of the uses of `LTOCodeGenerator` is to take it as a middle+back end. Sometimes
it is very helpful to access, especially get information from the optimized module.
If the information can be changed in optimization, it cannot be get before the
module is added to `LTOCodeGenerator`. This patch adds a function
`LTOCodeGenerator::getMergedModule` to access the `MergedModule`.

Reviewed By: steven_wu

Differential Revision: https://reviews.llvm.org/D114201