Support: Make VarStreamArrayIterator iterate over const values

VarStreamArrayIterator returns a reference to a just-computed internal
value. Change it to iterate over `const ValueType` to avoid allowing
clients to mutate the internal state, and to drop the
non-`const`-qualified operator*().

The removed operator*() was from 175d70e to get
iterator_facade_base::operator->() working, and this fixes the root
cause instead: setting `T` to `const ValueType` causes
iterator_facade_base to infer `PointerT` as `const ValueType*`.

Ironically, this is the last blocker for removing the const-incorrect
overload of `iterator_facade_base::operator->()`, whose presence
triggered adding the workaround in the first place :).

Differential Revision: https://reviews.llvm.org/D113797

Author: dexonsmith

llvm/include/llvm/Support/BinaryStreamArray.h

@@ -153,7 +153,7 @@ class VarStreamArray {
 template <typename ValueType, typename Extractor>
 class VarStreamArrayIterator
     : public iterator_facade_base<VarStreamArrayIterator<ValueType, Extractor>,
-                                  std::forward_iterator_tag, ValueType> {
+                                  std::forward_iterator_tag, const ValueType> {
   typedef VarStreamArrayIterator<ValueType, Extractor> IterType;
   typedef VarStreamArray<ValueType, Extractor> ArrayType;
 
@@ -197,11 +197,6 @@ class VarStreamArrayIterator
     return ThisValue;
   }
 
-  ValueType &operator*() {
-    assert(Array && !HasError);
-    return ThisValue;
-  }
-
   IterType &operator+=(unsigned N) {
     for (unsigned I = 0; I < N; ++I) {
       // We are done with the current record, discard it so that we are

llvm/tools/llvm-pdbutil/ExplainOutputStyle.cpp

@@ -373,7 +373,7 @@ static void explainDbiModiSubstreamOffset(LinePrinter &P, DbiStream &Dbi,
     ++Index;
   }
 
-  DbiModuleDescriptor &Descriptor = *Prev;
+  const DbiModuleDescriptor &Descriptor = *Prev;
   P.formatLine("which contains the descriptor for module {0} ({1}).", Index,
                Descriptor.getModuleName());
 }