IR: Add COMDATs to the IR

This new IR facility allows us to represent the object-file semantic of
a COMDAT group.

COMDATs allow us to tie together sections and make the inclusion of one
dependent on another. This is required to implement features like MS
ABI VFTables and optimizing away certain kinds of initialization in C++.

This functionality is only representable in COFF and ELF, Mach-O has no
similar mechanism.

Differential Revision: http://reviews.llvm.org/D4178

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211920 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/UniqueVector.h

@@ -22,13 +22,18 @@ namespace llvm {
 /// class should have an implementation of operator== and of operator<.
 /// Entries can be fetched using operator[] with the entry ID.
 template<class T> class UniqueVector {
+public:
+  typedef typename std::vector<T> VectorType;
+  typedef typename VectorType::iterator iterator;
+  typedef typename VectorType::const_iterator const_iterator;
+
 private:
   // Map - Used to handle the correspondence of entry to ID.
   std::map<T, unsigned> Map;
 
   // Vector - ID ordered vector of entries. Entries can be indexed by ID - 1.
   //
-  std::vector<T> Vector;
+  VectorType Vector;
 
 public:
   /// insert - Append entry to the vector if it doesn't already exist.  Returns
@@ -68,6 +73,18 @@ public:
     return Vector[ID - 1];
   }
 
+  /// \brief Return an iterator to the start of the vector.
+  iterator begin() { return Vector.begin(); }
+
+  /// \brief Return an iterator to the start of the vector.
+  const_iterator begin() const { return Vector.begin(); }
+
+  /// \brief Return an iterator to the end of the vector.
+  iterator end() { return Vector.end(); }
+
+  /// \brief Return an iterator to the end of the vector.
+  const_iterator end() const { return Vector.end(); }
+
   /// size - Returns the number of entries in the vector.
   ///
   size_t size() const { return Vector.size(); }