[clang][Interp][NFC] Add Pointer::isDereferencable()

We currently have a few places where we check whether a pointer can
be read from at all. Add a function to do that.

Author: tbaederr

clang/lib/AST/Interp/Pointer.h

@@ -584,6 +584,7 @@ class Pointer {
     assert(isLive() && "Invalid pointer");
     assert(isBlockPointer());
     assert(asBlockPointer().Pointee);
+    assert(isDereferencable());
     assert(Offset + sizeof(T) <=
            asBlockPointer().Pointee->getDescriptor()->getAllocSize());
 
@@ -603,6 +604,17 @@ class Pointer {
                                  sizeof(InitMapPtr))[I];
   }
 
+  /// Whether this block can be read from at all. This is only true for
+  /// block pointers that point to a valid location inside that block.
+  bool isDereferencable() const {
+    if (!isBlockPointer())
+      return false;
+    if (isPastEnd())
+      return false;
+
+    return true;
+  }
+
   /// Initializes a field.
   void initialize() const;
   /// Activats a field.