Add a test for @Lifetime(immortal)

Author: meg-gupta

include/swift/AST/DiagnosticsSema.def

@@ -7992,7 +7992,7 @@ ERROR(lifetime_dependence_cannot_infer_ambiguous_candidate, none,
        "Escapable",
        ())
 ERROR(lifetime_dependence_immortal_conflict_name, none,
-      "conflict between the parameter name and immortal keyword", ())
+      "conflict between the parameter name and 'immortal' contextual keyword", ())
 ERROR(lifetime_dependence_function_type, none,
       "lifetime dependencies on function types are not supported",
       ())

test/Sema/lifetime_attr.swift

@@ -39,3 +39,8 @@ func invalidDependence(_ x: consuming Klass) -> NE {
 func invalidTarget(_ result: inout NE, _ source: consuming NE) { // expected-error{{invalid duplicate target lifetime dependencies on function}}
   result = source
 }
+
+@lifetime(immortal)
+func immortalConflict(_ immortal: Int) -> NE { // expected-error{{conflict between the parameter name and 'immortal' contextual keyword}}
+  NE()
+}