[CodeCompletion] Add an already-fixed crashing test case

rdar://problem/58470999

Author: rintaro

validation-test/IDE/crashers_2_fixed/rdar58470999.swift

@@ -0,0 +1,9 @@
+// RUN: %target-swift-ide-test -code-completion -code-completion-token=COMPLETE -source-filename=%s
+
+protocol P {
+  func foo<T: P>(arg: T)
+}
+
+func foo(x: P) {
+  x.foo(arg: #^COMPLETE^#)
+}