[Serialization] Load non-public transitive dependencies on @testable imports

A @testable import allows a client to call internal decls which may
refer to non-public dependencies. To support such a use case, load
non-public transitive dependencies of a module when it's imported
@testable from the main module.

This replaces the previous behavior where we loaded those dependencies
for any modules built for testing. This was risky as we would load more
module for any debug build, opening the door to a different behavior
between debug and release builds. In contrast, applying this logic to
@testable clients will only change the behavior of test targets.

rdar://107329303

lib/Serialization/ModuleFile.cpp

@@ -133,6 +133,9 @@ ModuleFile::loadDependenciesForFileContext(const FileUnit *file,
 
   bool missingDependency = false;
   for (auto &dependency : Dependencies) {
+    if (forTestable && dependency.isLoaded())
+      continue;
+
     assert(!dependency.isLoaded() && "already loaded?");
 
     if (dependency.isHeader()) {
@@ -156,7 +159,7 @@ ModuleFile::loadDependenciesForFileContext(const FileUnit *file,
     }
 
     ModuleLoadingBehavior transitiveBehavior =
-      getTransitiveLoadingBehavior(dependency);
+      getTransitiveLoadingBehavior(dependency, forTestable);
 
     if (ctx.LangOpts.EnableModuleLoadingRemarks) {
       ctx.Diags.diagnose(diagLoc,
@@ -282,7 +285,8 @@ Status ModuleFile::associateWithFileContext(FileUnit *file, SourceLoc diagLoc,
 }
 
 ModuleLoadingBehavior
-ModuleFile::getTransitiveLoadingBehavior(const Dependency &dependency) const {
+ModuleFile::getTransitiveLoadingBehavior(const Dependency &dependency,
+    bool forTestable) const {
   ASTContext &ctx = getContext();
   ModuleDecl *mod = FileContext->getParentModule();
 
@@ -293,7 +297,8 @@ ModuleFile::getTransitiveLoadingBehavior(const Dependency &dependency) const {
   return Core->getTransitiveLoadingBehavior(dependency.Core,
                                             ctx.LangOpts.DebuggerSupport,
                                             isPartialModule,
-                                            ctx.LangOpts.PackageName);
+                                            ctx.LangOpts.PackageName,
+                                            forTestable);
 }
 
 bool ModuleFile::mayHaveDiagnosticsPointingAtBuffer() const {