Add default constructor to APSInt

- Creates uninitialized APInt. 
 - Prevents need for embedding arbitrary constants when used as an out
   parameter, for example.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54757 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/APSInt.h

@@ -23,6 +23,9 @@ namespace llvm {
 class APSInt : public APInt {
   bool IsUnsigned;
 public:
+  /// Default constructor that creates an uninitialized APInt.
+  explicit APSInt() {}
+
   /// APSInt ctor - Create an APSInt with the specified width, default to
   /// unsigned.
   explicit APSInt(uint32_t BitWidth, bool isUnsigned = true)