SIL: explicitly mark overloads as friends (NFC)

The class contains two inline operator overloads which are marked as
friends.  These overloads actually will be promoted to a global
function.  MSVC will not mark them as friends resulting in access to the
members being lost.  Explicitly mark the overloads as friends.  This is
needed by LLDB.

Author: compnerd

include/swift/SIL/SILInstruction.h

@@ -153,6 +153,9 @@ class SILInstructionResultArray {
 
   class iterator;
 
+  friend bool operator==(iterator, iterator);
+  friend bool operator!=(iterator, iterator);
+
   iterator begin() const;
   iterator end() const;