Make more use of llvm STLExtras (#4668)

This is essentially the result of looking at `.begin()` uses. We also
frequently do `std::shuffle`, but unfortunately STLExtras doesn't
provide a wrapper for that.

common/array_stack.h

@@ -78,7 +78,7 @@ class ArrayStack {
   auto AppendToTop(llvm::ArrayRef<ValueT> values) -> void {
     CARBON_CHECK(!array_offsets_.empty(),
                  "Must call PushArray before PushValues.");
-    values_.append(values.begin(), values.end());
+    llvm::append_range(values_, values);
   }
 
   // Returns the current number of values in all arrays.