[Frontend] Add flag to enable existential any

Author: angela-laar

include/swift/Basic/Features.def

@@ -93,6 +93,7 @@ SUPPRESSIBLE_LANGUAGE_FEATURE(NoAsyncAvailability, 340, "@available(*, noasync)"
 UPCOMING_FEATURE(ConciseMagicFile, 274, 6)
 UPCOMING_FEATURE(ForwardTrailingClosures, 286, 6)
 UPCOMING_FEATURE(BareSlashRegexLiterals, 354, 6)
+UPCOMING_FEATURE(ExistentialAny, 335, 6)
 
 EXPERIMENTAL_FEATURE(StaticAssert)
 EXPERIMENTAL_FEATURE(VariadicGenerics)

lib/AST/ASTPrinter.cpp

@@ -3009,6 +3009,10 @@ static bool usesFeatureConciseMagicFile(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureExistentialAny(Decl *decl) {
+  return false;
+}
+
 static bool usesFeatureForwardTrailingClosures(Decl *decl) {
   return false;
 }