[NFC] Add a slow type checker test involving string concatenation.

Author: hborla

validation-test/Sema/type_checker_perf/slow/rdar74035425.swift

@@ -0,0 +1,12 @@
+// RUN: %target-typecheck-verify-swift -solver-expression-time-threshold=1
+// REQUIRES: tools-release,no_asan
+
+struct Value {
+  let debugDescription: String
+}
+
+func test(values: [[Value]]) -> String {
+  // expected-error@+1 {{the compiler is unable to type-check this expression in reasonable time}}
+  "[" + "" + values.map({ "[" + $0.map({ $0.debugDescription }).joined(separator: ", ") + "]" }).joined(separator: ", ") + "]"
+}
+