Factor out a Pattern sum type from Expression (#685)

`Pattern` is intended to pilot some changes I would like to apply to all our sum types:
- The alternatives are expressed as derived classes rather than members of a `std::variant`.
- The alternatives are classes in the [style guide sense](https://google.github.io/styleguide/cppguide.html#Structs_vs._Classes), meaning they can have invariants, but can't have public data members.
- Creating an object is expressed using a constructor rather than a factory function.
- Accessing an alternative is expressed as a cast (using LLVM's RTTI system) rather than `std::get` or a `Get` method.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/testdata/fun6_fail_type.golden

@@ -1,2 +1,4 @@
-COMPILATION ERROR: 10: tuples of different length
+COMPILATION ERROR: 10: type error in call
+expected: (0 = Int, 1 = Int)
+actual: (0 = (0 = Int, 1 = Int))
 EXIT CODE: 255

executable_semantics/testdata/match_type.carbon

@@ -1,17 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-
-fn main() -> Int {
-  var t: auto = fnty (Int,Int);
-  var x: Int = 0;
-  match (t) {
-    case z: Type =>
-      x = x + 1;
-  }
-  match (t) {
-    case fnty(a: Type,b: Type) =>
-      x = x - 1;
-  }
-  return x;
-}

executable_semantics/testdata/match_type.golden

@@ -1 +0,0 @@
-result: 0

executable_semantics/testdata/pattern_variable_fail.golden

@@ -1,2 +1,2 @@
-COMPILATION ERROR: 7: pattern variables are only allowed in pattern context
+executable_semantics/testdata/pattern_variable_fail.carbon:7: syntax error, unexpected :
 EXIT CODE: 255