[ADT] Delegate among DenseMap constructors (NFC) (#168309)

This patch teaches DenseMap constructors to delegate to other DenseMap
constructors where we can.

The intent is for these constructors to build on top of a higher-level
concept like the default-constructed instance instead of calling init
on our own.

This is part of the effort outlined in #168255.

Author: kazutakahirata

llvm/include/llvm/ADT/DenseMap.h

@@ -754,18 +754,12 @@ class DenseMap : public DenseMapBase<DenseMap<KeyT, ValueT, KeyInfoT, BucketT>,
     init(NumElementsToReserve);
   }
 
-  DenseMap(const DenseMap &other) : BaseT() {
-    init(0);
-    this->copyFrom(other);
-  }
+  DenseMap(const DenseMap &other) : DenseMap() { this->copyFrom(other); }
 
-  DenseMap(DenseMap &&other) : BaseT() {
-    init(0);
-    this->swap(other);
-  }
+  DenseMap(DenseMap &&other) : DenseMap() { this->swap(other); }
 
-  template <typename InputIt> DenseMap(const InputIt &I, const InputIt &E) {
-    init(std::distance(I, E));
+  template <typename InputIt>
+  DenseMap(const InputIt &I, const InputIt &E) : DenseMap(std::distance(I, E)) {
     this->insert(I, E);
   }
 
@@ -908,19 +902,15 @@ class SmallDenseMap
     init(NumElementsToReserve);
   }
 
-  SmallDenseMap(const SmallDenseMap &other) : BaseT() {
-    init(0);
+  SmallDenseMap(const SmallDenseMap &other) : SmallDenseMap() {
     this->copyFrom(other);
   }
 
-  SmallDenseMap(SmallDenseMap &&other) : BaseT() {
-    init(0);
-    this->swap(other);
-  }
+  SmallDenseMap(SmallDenseMap &&other) : SmallDenseMap() { this->swap(other); }
 
   template <typename InputIt>
-  SmallDenseMap(const InputIt &I, const InputIt &E) {
-    init(std::distance(I, E));
+  SmallDenseMap(const InputIt &I, const InputIt &E)
+      : SmallDenseMap(std::distance(I, E)) {
     this->insert(I, E);
   }