[Test] Add a slow type checker test case from source compatibility suite.

Author: hborla

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

@@ -0,0 +1,12 @@
+// RUN: %target-typecheck-verify-swift -swift-version 5 -solver-expression-time-threshold=1
+
+func method(_ arg: String, body: () -> [String]) {}
+
+func test(str: String, properties: [String]) {
+  // expected-error@+1 {{the compiler is unable to type-check this expression in reasonable time}}
+  method(str + "" + str + "") {
+    properties.map { param in
+      "" + param + "" + param + ""
+    } + [""]
+  }
+}