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

llvm/include/llvm/IR/Dominators.h

@@ -277,6 +277,12 @@ struct DominatorTreeVerifierPass : PassInfoMixin<DominatorTreeVerifierPass> {
   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
 };
 
+/// Enables verification of dominator trees.
+///
+/// This check is expensive and is disabled by default.  `-verify-dom-info`
+/// allows selectively enabling the check without needing to recompile.
+extern bool VerifyDomInfo;
+
 /// Legacy analysis pass which computes a \c DominatorTree.
 class DominatorTreeWrapperPass : public FunctionPass {
   DominatorTree DT;

llvm/include/llvm/Support/Debug.h

@@ -85,10 +85,6 @@ extern bool DebugFlag;
 /// turning the checks on without need to recompile.
 /// \{
 
-/// Enables verification of dominator trees.
-///
-extern bool VerifyDomInfo;
-
 /// Enables verification of loop info.
 ///
 extern bool VerifyLoopInfo;