[Serialization] Preserve @_implementationOnly through module merging

When we build incrementally, we produce "partial swiftmodules" for
each input source file, then merge them together into the final
compiled module that, among other things, gets used for debugging.
Without this, we'd drop @_implementationOnly imports and any types
from the modules that were imported during the module-merging step
and then be unable to debug those types

lib/Serialization/ModuleFile.cpp

@@ -1460,7 +1460,8 @@ ModuleFile::ModuleFile(
 }
 
 Status ModuleFile::associateWithFileContext(FileUnit *file,
-                                            SourceLoc diagLoc) {
+                                            SourceLoc diagLoc,
+                                            bool treatAsPartialModule) {
   PrettyStackTraceModuleFile stackEntry(*this);
 
   assert(getStatus() == Status::Valid && "invalid module file");
@@ -1513,7 +1514,7 @@ Status ModuleFile::associateWithFileContext(FileUnit *file,
       continue;
     }
 
-    if (dependency.isImplementationOnly())
+    if (dependency.isImplementationOnly() && !treatAsPartialModule)
       continue;
 
     StringRef modulePathStr = dependency.RawPath;