Support: Make VarStreamArrayIterator::operator*() const-qualified

[dexonsmith]

Summary

VarStreamArrayIterator returns a reference to a just-computed internal
value; since it's an iterator over ValueType, change operator*() to
always return ValueType&.

This patch is conservative in case it's useful to std::move() out an
extracted value...

But another option is to change it to iterate over const ValueType
and always return const ValueType&:

... : iterator_facade_base<..., const ValueType>

It only requires a one-line change to ExplainOutputStyle.cpp to get tests
compiling and passing:

-  DbiModuleDescriptor &Descriptor = *Prev;
+  const DbiModuleDescriptor &Descriptor = *Prev;

Based on 175d70ee5c2f03f640151488f5f33b7bd9b96f8d, perhaps that would
better match the original intent? @amccarth, @zturner, thoughts?

[dexonsmith]

Since Phab didn't seem to turn 175d70ee5c2f03f640151488f5f33b7bd9b96f8d into a link, that was reviewed in https://reviews.llvm.org/D32235.

[dexonsmith]

Just posted the alternative at https://reviews.llvm.org/D113797. It seems cleaner to me; basically a fix-up for 175d70ee5c2f03f640151488f5f33b7bd9b96f8d.

[dblaikie]

Yeah, I'd take the const ValueType option, with the one extra fix.

[dblaikie]

In D113796#3128512, @dblaikie wrote:

Yeah, I'd take the const ValueType option, with the one extra fix.

Oh, I see you posted that separately in D113797 so approving that one instead.

llvm/include/llvm/Support/BinaryStreamArray.h

bool operator==(const IterType &R) const {
     // Both iterators are at the end.
     if (!Array && !R.Array)
       return true;
 
     // One is not at the end and one is.
     return false;
   }
 
-  const ValueType &operator*() const {
-    assert(Array && !HasError);
-    return ThisValue;
-  }
-
-  ValueType &operator*() {
+  ValueType &operator*() const {
     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
       // positioned at the next record.
private:
   }
   void markError() {
     moveToEnd();
     HasError = true;
     if (HadError != nullptr)
       *HadError = true;
   }
 
-  ValueType ThisValue;
+  mutable ValueType ThisValue;
   BinaryStreamRef IterRef;
   Extractor Extract;
   const ArrayType *Array{nullptr};
   uint32_t ThisLen{0};
   uint32_t AbsOffset{0};
   bool HasError{false};
   bool *HadError{nullptr};
 };