Merge pull request #40282 from hborla/existential-any

[SE-0335] Introduce existential `any`

include/swift/Basic/LangOptions.h

@@ -310,6 +310,10 @@ namespace swift {
     /// `func f() -> <T> T`.
     bool EnableExperimentalNamedOpaqueTypes = false;
 
+    /// Enable support for explicit existential types via the \c any
+    /// keyword.
+    bool EnableExplicitExistentialTypes = false;
+
     /// Enable experimental flow-sensitive concurrent captures.
     bool EnableExperimentalFlowSensitiveConcurrentCaptures = false;