[Small]BitVector::reference: Explicitly default copy construction as it is deprecated in C++11 in the presence of explicit copy assignment.

I tried making these private & friended to the BitVector, but that
didn't work - there's one use of BitVector::reference in Clang that
actually copies it into a local variable & uses it from there, rather
than just using the result of op[] in a temporary expression.

Whether or not this is desired is debatable (we could just fix that one
use in Clang) & it's not clear which way the C++ standard falls on this
for std::bitset's reference type (it has the same bug at least in
libstdc++, but Clang's -Wdeprecated doesn't flag it, because it's in a
standard header)

While it was only BitVector::reference's copy ctor that was referenced
by user code, I made SmallBitVector::reference's copy ctor public too,
for consistency.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231099 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/BitVector.h

@@ -53,7 +53,7 @@ public:
       BitPos = Idx % BITWORD_SIZE;
     }
 
-    ~reference() {}
+    reference(const reference&) = default;
 
     reference &operator=(reference t) {
       *this = bool(t);

include/llvm/ADT/SmallBitVector.h

@@ -66,6 +66,8 @@ public:
   public:
     reference(SmallBitVector &b, unsigned Idx) : TheVector(b), BitPos(Idx) {}
 
+    reference(const reference&) = default;
+
     reference& operator=(reference t) {
       *this = bool(t);
       return *this;