Reduce buildBlock overloads with pairwise buildBlock(combining:into:).

This patch replaces `O(2^arity)` overloads of `buildBlock` with `O(arity^2)` overloads of `buildBlock(combining:into:)` with the language feature implemented in swiftlang/swift#40799. This improves library compilation time and fixes an issue where nested captures did not have a flattened type. This also allows us to switch back to native tuples for `Match`, which will be addressed in a follow-up patch.

Author: rxwei

Sources/_StringProcessing/RegexDSL/DSLTree.swift

@@ -493,4 +493,11 @@ extension DSLTree.Node {
     }
     return .concatenation([self, newNode])
   }
+
+  func appendingAlternationCase(_ newNode: DSLTree.Node) -> DSLTree.Node {
+    if case .alternation(let components) = self {
+      return .alternation(components + [newNode])
+    }
+    return .alternation([self, newNode])
+  }
 }