implemented parsing and typechecking for @actorIndependent(unsafe)

[broken] first impl of @actorIndependent in the type checker.

[broken] fixed mistake in my parsing code wrt invalid source range

[broken] found another spot where ActorIndependent needs custom handling

[broken] incomplete set of @actorIndependent(unsafe) tests

updates to ActorIndependentUnsafe

[fixed] add FIXME plus simple handling of IndependentUnsafe context

finished @actorIndependent(unsafe) regression tests

added wip serialization / deserialization test

focus test to just one actor class

round-trip serialize/deserialize test for @actorIndependent

serialize -> deserialize -> serialize -> compare to original

most of doug's comments

addressed robert's comments

fix printing bug; add module printing to regression test

[nfc] update comment for ActorIsolation::IndependentUnsafe

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 = 582; // sil specialize attribute module parameter
+const uint16_t SWIFTMODULE_VERSION_MINOR = 583; // @actorIndependent safe/unsafe attribute
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///
@@ -1807,6 +1807,11 @@ namespace decls_block {
     BCFixed<2>  // inline value
   >;
 
+  using ActorIndependentDeclAttrLayout = BCRecordLayout<
+    ActorIndependent_DECL_ATTR,
+    BCFixed<1>  // unsafe flag
+  >;
+
   using OptimizeDeclAttrLayout = BCRecordLayout<
     Optimize_DECL_ATTR,
     BCFixed<2>  // optimize value