Imported modules only import the stdlib if it exists.

Any configuration where the stdlib doesn't exist is an atypical one,
but being more permissive here lets us successfully import many kinds
of declarations even without a stdlib.  That in turn makes it possible
to write IRGen tests that need foreign types and exercise a specific
target without restricting the test to only run when building that
target.

Author: rjmccall

lib/ClangImporter/ClangImporter.cpp

@@ -3082,7 +3082,8 @@ void ClangModuleUnit::getImportedModules(
   switch (filter) {
   case ModuleDecl::ImportFilter::All:
   case ModuleDecl::ImportFilter::Private:
-    imports.push_back({ModuleDecl::AccessPathTy(), owner.getStdlibModule()});
+    if (auto stdlib = owner.getStdlibModule())
+      imports.push_back({ModuleDecl::AccessPathTy(), stdlib});
     break;
   case ModuleDecl::ImportFilter::Public:
     break;