[Serialization] Delay all actions in the same module together. (#8123)

Back in December DougG added code to delay the formation of generic
environments until all declarations from a particular module had been
deserialized, to avoid circular dependencies caused by too-eager
deserialization of protocol members. This worked great for fully-built
modules, but still had some problems with module merging, the phase of
multi-file compilation where the "partial" swiftmodules that
correspond to each source file in a target are loaded and remitted as
a single swiftmodule. Fix this by picking one of the partial
swiftmodules as the representative one for delayed actions, and wait
until deserialization is complete for /all/ of the serialized ASTs in
the same target to form the generic environments.

rdar://problem/30984417

include/swift/Serialization/ModuleFile.h

@@ -83,6 +83,9 @@ class ModuleFile : public LazyMemberLoader {
   /// Declaration contexts with delayed generic environments, which will be
   /// completed as a pending action.
   ///
+  /// This should only be used on the module returned by
+  /// getModuleFileForDelayedActions().
+  ///
   /// FIXME: This is needed because completing a generic environment can
   /// require the type checker, which might be gone if we delay generic
   /// environments too far. It is a hack.
@@ -93,7 +96,8 @@ class ModuleFile : public LazyMemberLoader {
     ModuleFile &MF;
 
   public:
-    DeserializingEntityRAII(ModuleFile &MF) : MF(MF) {
+    DeserializingEntityRAII(ModuleFile &mf)
+        : MF(mf.getModuleFileForDelayedActions()) {
       ++MF.NumCurrentDeserializingEntities;
     }
     ~DeserializingEntityRAII() {
@@ -108,6 +112,13 @@ class ModuleFile : public LazyMemberLoader {
   };
   friend class DeserializingEntityRAII;
 
+  /// Picks a specific ModuleFile instance to serve as the "delayer" for the
+  /// entire module.
+  ///
+  /// This is usually \c this, but when there are partial swiftmodules all
+  /// loaded for the same module it may be different.
+  ModuleFile &getModuleFileForDelayedActions();
+
   /// Finish any pending actions that were waiting for the topmost entity to
   /// be deserialized.
   void finishPendingActions();