[SE-0470] Prohibit isolated conformances in dynamic casts marked as such

Certain dynamic casts cannot work safely with isolated conformances,
regardless of what executor the code runs on. For such cases, reject
all attempts to conform to the type.

include/swift/ABI/MetadataValues.h

@@ -313,6 +313,11 @@ enum class DynamicCastFlags : size_t {
   /// True if the cast should destroy the source value on failure;
   /// false if the value should be left in place.
   DestroyOnFailure = 0x4,
+
+  /// True if the cast should prohibit the use of any isolated conformances,
+  /// for example because there is a Sendable constraint on the existential
+  /// type we're casting to.
+  ProhibitIsolatedConformances = 0x8,
 };
 inline bool operator&(DynamicCastFlags a, DynamicCastFlags b) {
   return (size_t(a) & size_t(b)) != 0;