[llvm] [NFC] Move DroppedVariableStats to its own file and redesign it to be extensible. (PR #115563)

[Adrian Prantl via llvm-commits]

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+///===- DroppedVariableStats.cpp - Opt Diagnostic -*- C++ -*---------------===//
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adrian-prantl wrote:

nit: -*- C++ -*- marker is only needed for `.h` files where it's ambiguous whether it is C++, C or ObjC.

https://github.com/llvm/llvm-project/pull/115563