As part of the unified IR refactor.
This PR refactors the target codegen to use IRModule containing tir::PrimFuncs.

In order to break the refactor into several steps without breaking the codebase,
we built an conversion pass to convert Array<LoweredFunc> into IRModule.

The follow-up refactors will gradually move the passes covered by IRModule up
until we cover all the passes. Then we can remove the additional redundant
concepts such as LoweredFunc.