Add iterator for initializers in PatternBindingDecl

This makes it safer and easier to iterate over the initializer
expression in the pattern binding decl. I look forward to refactoring
all the places running an index loop over the `getNumPatternEntries`.

Author: etcwilde

include/swift/AST/Decl.h

@@ -1682,6 +1682,62 @@ class PatternBindingDecl final : public Decl,
     return const_cast<PatternBindingDecl*>(this)->getMutablePatternList();
   }
 
+  /// Clean up walking the initializers for the pattern
+  class InitIterator {
+
+    const PatternBindingDecl &decl;
+    unsigned currentPatternEntryIndex;
+
+    void next() { ++currentPatternEntryIndex; }
+
+  public:
+    using value_type = Expr *;
+    using pointer = value_type;
+    using reference = value_type;
+    using difference_type = unsigned;
+
+    InitIterator(const PatternBindingDecl &decl, unsigned start = 0)
+        : decl(decl), currentPatternEntryIndex(start) {}
+
+    InitIterator &operator++() {
+      next();
+      return *this;
+    }
+
+    InitIterator operator++(int) {
+      InitIterator newIterator(decl, currentPatternEntryIndex);
+      newIterator.next();
+      return newIterator;
+    }
+
+    pointer operator->() { return decl.getInit(currentPatternEntryIndex); }
+
+    pointer operator*() { return decl.getInit(currentPatternEntryIndex); }
+
+    difference_type operator-(const InitIterator &other) {
+      return currentPatternEntryIndex - other.currentPatternEntryIndex;
+    }
+
+    bool operator==(const InitIterator &other) const {
+      return &decl == &other.decl &&
+             currentPatternEntryIndex == other.currentPatternEntryIndex;
+    }
+
+    bool operator!=(const InitIterator &other) const {
+      return !(*this == other);
+    }
+  };
+
+  InitIterator beginInits() const { return InitIterator(*this); }
+
+  InitIterator endInits() const {
+    return InitIterator(*this, getNumPatternEntries());
+  }
+
+  llvm::iterator_range<InitIterator> initializers() const {
+    return llvm::make_range(beginInits(), endInits());
+  }
+
   void setInitStringRepresentation(unsigned i, StringRef str) {
     getMutablePatternList()[i].setInitStringRepresentation(str);
   }