[MLIR][LLVM] Move the LLVM inliner interface into a separate file.

A fully fledged LLVM inliner will require a lot of logic. Since
`LLVMDialect.cpp` is large enough as it is, preemptively outline the
inlining logic into a separate `.cpp` file. This will also allow us to
add a `DEBUG_TYPE` for debugging the inliner.

The name `LLVMInlining` was chosen over `LLVMInlinerInterface` to keep
the option open for exposing inlining functionality even when not
invoked through the `DialectInlinerInterface`.

Depends on D146616

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D146628