[field-pruned-liveness] Add two helper constructors to TypeTreeLeafTypeRange.

The first is a default constructor that initializes both startEltOffset and
endEltOffset to 0. This is used to ensure that we can use certain standard
algorithms that expect to be able to perform a default constructor when
inserting an initial value into a collection.

The second adds the ability to construct a TypeTreeLeafTypeRange for a
SILType/SILFunction if one has a type without an actual value.

Author: gottesmm

include/swift/SIL/FieldSensitivePrunedLiveness.h

@@ -213,15 +213,20 @@ struct TypeTreeLeafTypeRange {
   SubElementNumber startEltOffset;
   SubElementNumber endEltOffset;
 
-private:
+public:
+  TypeTreeLeafTypeRange() : startEltOffset(0), endEltOffset(0) {}
+
   TypeTreeLeafTypeRange(SubElementNumber start, SubElementNumber end)
       : startEltOffset(start), endEltOffset(end) {}
 
-public:
   /// The leaf type range for the entire type tree.
   TypeTreeLeafTypeRange(SILValue rootAddress)
       : startEltOffset(0), endEltOffset(TypeSubElementCount(rootAddress)) {}
 
+  /// The leaf type range for the entire type tree.
+  TypeTreeLeafTypeRange(SILType rootType, SILFunction *fn)
+      : startEltOffset(0), endEltOffset(TypeSubElementCount(rootType, fn)) {}
+
   /// The leaf type sub-range of the type tree of \p rootAddress, consisting of
   /// \p projectedAddress and all of \p projectedAddress's descendent fields in
   /// the type tree.