test: add a syntax parse round-trip test

Author: nkcsgexi

lit_tests/print_verify_tree.swift

@@ -0,0 +1,11 @@
+// RUN: %empty-directory(%t)
+// RUN: %lit-test-helper -print-tree -source-file %s > %t.result
+// RUN: diff -u %t.result %s
+
+func foo() {
+#if swift(>=3.2)
+    components.append("-b \"\(string[..<string.index(before: string.endIndex)])\"")
+#else
+    components.append("-b \"\(string.substring(to: string.characters.index(before: string.endIndex)))\"")
+#endif
+}