This patch attempts to reland
https://github.com/llvm/llvm-project/pull/120780 while addressing the
issues that caused the patch to be reverted.

Namely:

1. The patch had included code from the llvm/Passes directory in the
llvm/CodeGen directory.

2. The patch increased the backend compile time by 2% due to adding a
very expensive include in MachineFunctionPass.h

The patch has been re-structured so that there is no dependency between
the llvm/Passes and llvm/CodeGen directory, by moving the base class,
`class DroppedVariableStats` to the llvm/IR directory.

The expensive include in MachineFunctionPass.h has been changed to
contain forward declarations instead of other header includes which was
pulling a ton of code into MachineFunctionPass.h and should resolve any
issues when it comes to compile time increase.

This reverts commit 3bf91ad2a9c75dd045961e45fdd830fd7b7a5455.
(llvmorg-20-init-16123-g3bf91ad2a9c7)

`llvm/CodeGen` should not depend on `llvm/Passes`.

This patch uses the DroppedVariableStats class to add dropped variable
statistics for MIR passes.

Reland 1c082c9cd12efaa67a32c5da89a328c458ed51c5