[NFC] Add DebugVariableAggregate class

[Orlando]

Summary

A DebugVariableAggregate is a DebugVariable that discards FragmentInfo; it represents a whole variable instance.

[StephenTozer]

I believe there are a few places in the existing codebase where we intentionally drop fragments that could be updated to use this class; on the whole though, I approve of using a new class to differentiate between a variable fragment that happens to be the whole variable vs an explicit reference to the whole variable regardless of fragmentation.

llvm/include/llvm/IR/DebugInfoMetadata.h

static unsigned getHashValue(const DebugVariable &D) {
     return hash_combine(D.getVariable(), HV, D.getInlinedAt());
   }
 
   static bool isEqual(const DebugVariable &A, const DebugVariable &B) {
     return A == B;
   }
 };
 
+/// Identifies a unique instance of a whole variable (discards/ignores fragment
+/// information).
+class DebugVariableAggregate : public DebugVariable {
+public:
+  DebugVariableAggregate(const DbgVariableIntrinsic *DVI);
+  DebugVariableAggregate(const DebugVariable &V)
+      : DebugVariable(V.getVariable(), std::nullopt, V.getInlinedAt()) {}
+};
+
+template <>
+struct DenseMapInfo<DebugVariableAggregate>
+    : public DenseMapInfo<DebugVariable> {};
 } // end namespace llvm
 
 #undef DEFINE_MDNODE_GET_UNPACK_IMPL
 #undef DEFINE_MDNODE_GET_UNPACK
 #undef DEFINE_MDNODE_GET
 
 #endif // LLVM_IR_DEBUGINFOMETADATA_H

llvm/lib/IR/DebugInfoMetadata.cpp

 const DIExpression::FragmentInfo DebugVariable::DefaultFragment = {
     std::numeric_limits<uint64_t>::max(), std::numeric_limits<uint64_t>::min()};
 
 DebugVariable::DebugVariable(const DbgVariableIntrinsic *DII)
     : Variable(DII->getVariable()),
       Fragment(DII->getExpression()->getFragmentInfo()),
       InlinedAt(DII->getDebugLoc().getInlinedAt()) {}
 
+DebugVariableAggregate::DebugVariableAggregate(const DbgVariableIntrinsic *DVI)
+    : DebugVariable(DVI->getVariable(), std::nullopt,
+                    DVI->getDebugLoc()->getInlinedAt()) {}
+
 DILocation::DILocation(LLVMContext &C, StorageType Storage, unsigned Line,
                        unsigned Column, ArrayRef<Metadata *> MDs,
                        bool ImplicitCode)
     : MDNode(C, DILocationKind, Storage, MDs) {
   assert((MDs.size() == 1 || MDs.size() == 2) &&
          "Expected a scope and optional inlined-at");
 
   // Set line and column.