IR: move the declaration of `VerifyDomInfo` (NFC)

This moves the declaration of `VerifyDomInfo` into
`llvm/IR/Dominators.h` from `llvm/Support/Debug.h`.  Although this is a
debugging utility, the definition of the symbol is in LLVMIR, not in
LLVMSupport.  This moves the declaration to the containing modules'
header.

Reviewed By: rnk, mehdhi_amini

Differential Revision: https://reviews.llvm.org/D109395