[NFC] Remove "Runtime" from this feature name to avoid confusion

Author: rjmccall

include/swift/AST/FeatureAvailability.def

@@ -18,6 +18,13 @@
 /// FEATURE(N, V)
 ///   N - The name of the feature (in UpperCamelCase).
 ///   V - The Swift version number, as a tuple, or FUTURE.
+///
+/// The feature Foo turns into two methods on ASTContext:
+/// getFooRuntimeAvailability(), which returns the exact version provided
+/// here, and getFooAvailability(), which maps that version into a version
+/// of the target OS.  Because both of these methods exist, it is a bad idea
+/// to use a feature name that ends in "Runtime", or people might get very
+/// confused.
 #ifndef FEATURE
 #define FEATURE(N, V)
 #endif
@@ -48,7 +55,8 @@ FEATURE(Concurrency,                                    (5, 5))
 FEATURE(MultiPayloadEnumTagSinglePayload,               (5, 6))
 FEATURE(ObjCIsUniquelyReferenced,                       (5, 6))
 
-FEATURE(ParameterizedExistentialRuntime,                (5, 7))
+// Metadata and casting support for parameterized existential types
+FEATURE(ParameterizedExistential,                       (5, 7))
 
 FEATURE(VariadicGenericType,                            (5, 9))
 FEATURE(SignedConformsToProtocol,                       (5, 9))

lib/IRGen/IRGenModule.cpp

@@ -960,7 +960,7 @@ namespace RuntimeConstants {
   }
 
   RuntimeAvailability ParameterizedExistentialAvailability(ASTContext &Context) {
-    auto featureAvailability = Context.getParameterizedExistentialRuntimeAvailability();
+    auto featureAvailability = Context.getParameterizedExistentialAvailability();
     if (!isDeploymentAvailabilityContainedIn(Context, featureAvailability)) {
       return RuntimeAvailability::ConditionallyAvailable;
     }

lib/Sema/TypeCheckAvailability.cpp

@@ -2988,7 +2988,7 @@ bool swift::diagnoseParameterizedProtocolAvailability(
     SourceRange ReferenceRange, const DeclContext *ReferenceDC) {
   return TypeChecker::checkAvailability(
       ReferenceRange,
-      ReferenceDC->getASTContext().getParameterizedExistentialRuntimeAvailability(),
+      ReferenceDC->getASTContext().getParameterizedExistentialAvailability(),
       diag::availability_parameterized_protocol_only_version_newer,
       ReferenceDC);
 }