[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

lib/Serialization/ModuleFile.cpp

@@ -1279,7 +1279,11 @@ Status ModuleFile::associateWithFileContext(FileUnit *file,
   return getStatus();
 }
 
-ModuleFile::~ModuleFile() = default;
+ModuleFile::~ModuleFile() {
+  assert(DelayedGenericEnvironments.empty() &&
+         "either finishPendingActions() was never called, or someone forgot "
+         "to use getModuleFileForDelayedActions()");
+}
 
 void ModuleFile::lookupValue(DeclName name,
                              SmallVectorImpl<ValueDecl*> &results) {