Add the @exclusivity attribute.

The `@exclusivity(unchecked)` attribute can be used on variables to selectively disable exclusivity checking.
For completeness, also the `@exclusivity(checked)` variant is supported: it turns on exclusivity checking for specific variables if exclusivity enforcement is disabled by the command line option.

This new attribute is a missing implementation part of SE-0176 (https://github.com/apple/swift-evolution/blob/main/proposals/0176-enforce-exclusive-access-to-memory.md).

rdar://31121356

lib/Serialization/Serialization.cpp

@@ -2520,6 +2520,14 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
       return;
     }
 
+    case DAK_Exclusivity: {
+      auto *theAttr = cast<ExclusivityAttr>(DA);
+      auto abbrCode = S.DeclTypeAbbrCodes[ExclusivityDeclAttrLayout::Code];
+      ExclusivityDeclAttrLayout::emitRecord(S.Out, S.ScratchRecord, abbrCode,
+                                            (unsigned)theAttr->getMode());
+      return;
+    }
+
     case DAK_Effects: {
       auto *theAttr = cast<EffectsAttr>(DA);
       auto abbrCode = S.DeclTypeAbbrCodes[EffectsDeclAttrLayout::Code];