[Inliner][NFC] silence gcc 'overloaded-virtual' warning on hiding of Pass::doInitialization

When compiling with -Werror=overloaded-virtual, gcc emits this:
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llvm/include/llvm/Pass.h:102:16: error: ‘virtual bool llvm::Pass::doInitialization(llvm::Module&)’ was hidden [-Werror=overloaded-virtual]
   virtual bool doInitialization(Module &)  { return false; }
                ^~~~~~~~~~~~~~~~
In file included from llvm/lib/Transforms/IPO/Inliner.cpp:20:0:
llvm/include/llvm/Transforms/IPO/Inliner.h:38:8: error:   by ‘virtual bool llvm::LegacyInlinerBase::doInitialization(llvm::CallGraph&)’ [-Werror=overloaded-virtual]
   bool doInitialization(CallGraph &CG) override;
        ^~~~~~~~~~~~~~~~
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This is an old issue which has just started biting our downstream after
a slight rearrangement of includes around Inliner.
Fixing it similar to how doFinalization was done years ago.