[clangd] Use capacity() instead of size() in RefSlab::bytes()

[nridge]

This was suggested in this comment.

[gribozavr]

Do you have commit access?

[nridge]

I do not.

[ilya-biryukov]

I'll take the courtesy and land it since @gribozavr is OOO today.

clang-tools-extra/clangd/index/Ref.h

public:
   const_iterator end() const { return Refs.end(); }
   /// Gets the number of symbols.
   size_t size() const { return Refs.size(); }
   size_t numRefs() const { return NumRefs; }
   bool empty() const { return Refs.empty(); }
 
   size_t bytes() const {
     return sizeof(*this) + Arena.getTotalMemory() +
-           sizeof(value_type) * Refs.size();
+           sizeof(value_type) * Refs.capacity();
   }
 
   /// RefSlab::Builder is a mutable container that can 'freeze' to RefSlab.
   class Builder {
   public:
     Builder() : UniqueStrings(Arena) {}
     /// Adds a ref to the slab. Deep copy: Strings will be owned by the slab.
     void insert(const SymbolID &ID, const Ref &S);