[clangd] NFC: mark single-parameter constructors explicit

[kbobyrev]

Summary

Code health: prevent implicit conversions to user-defined types.

clang-tools-extra/clangd/index/dex/Iterator.cpp

 /// tree) and is simply a wrapper around PostingList::const_iterator.
 class DocumentIterator : public Iterator {
 public:
-  DocumentIterator(PostingListRef Documents)
+  explicit DocumentIterator(PostingListRef Documents)
       : Documents(Documents), Index(std::begin(Documents)) {}
 
   bool reachedEnd() const override { return Index == std::end(Documents); }

 /// iterator restores the invariant: all children must point to the same item.
 class AndIterator : public Iterator {
 public:
-  AndIterator(std::vector<std::unique_ptr<Iterator>> AllChildren)
+  explicit AndIterator(std::vector<std::unique_ptr<Iterator>> AllChildren)
       : Children(std::move(AllChildren)) {
     assert(!Children.empty() && "AND iterator should have at least one child.");
     // Establish invariants.

 /// soon as all of its children are exhausted.
 class OrIterator : public Iterator {
 public:
-  OrIterator(std::vector<std::unique_ptr<Iterator>> AllChildren)
+  explicit OrIterator(std::vector<std::unique_ptr<Iterator>> AllChildren)
       : Children(std::move(AllChildren)) {
     assert(Children.size() > 0 && "OR iterator must have at least one child.");
   }

 /// in O(1).
 class TrueIterator : public Iterator {
 public:
-  TrueIterator(DocID Size) : Size(Size) {}
+  explicit TrueIterator(DocID Size) : Size(Size) {}
 
   bool reachedEnd() const override { return Index >= Size; }