[Concurrency] Add support for @GlobalActor(unsafe).

Allow us to tag declarations that are meant to be in a global actor, but
for which we don't yet want to enforce everything. This will be used for
better staging-in of global actor annotations, but for now it's a fancy
way to document @actorIndependent(unsafe).

Stages in the syntax for rdar://74241687 without really implementing it.

lib/Serialization/ModuleFormat.h

@@ -56,7 +56,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 599; // has-async-alternative
+const uint16_t SWIFTMODULE_VERSION_MINOR = 600; // custom attr (unsafe)
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///
@@ -1943,9 +1943,9 @@ namespace decls_block {
   using CustomDeclAttrLayout = BCRecordLayout<
     Custom_DECL_ATTR,
     BCFixed<1>,  // implicit flag
-    TypeIDField // type referenced by this custom attribute
+    TypeIDField, // type referenced by this custom attribute
+    BCFixed<1>   // is the argument (unsafe)
   >;
-
 }
 
 /// Returns the encoding kind for the given decl.