Don't treat missing imports as a fatal error in REPL mode.

We don't want typos in import statements to take down the whole REPL, but we
/do/ want the REPL to be honoring fatal errors that effectively take down the
ASTContext.

This doesn't (yet) apply to the real LLDB REPL, which does not use
SourceFileKind::REPL for its input. The right option to test there is
LangOpts.DebuggerSupport, but that's currently being set for Playgrounds as
well. I've filed <rdar://problem/18090611> for LLDB to adjust their input.

Part of <rdar://problem/17994094>

Swift SVN r21383

lib/Serialization/SerializedModuleLoader.cpp

@@ -299,7 +299,7 @@ Module *SerializedModuleLoader::loadModule(SourceLoc importLoc,
                                          isFramework)) {
       if (err != std::errc::no_such_file_or_directory) {
         Ctx.Diags.diagnose(moduleID.second, diag::sema_opening_import,
-                           moduleID.first.str(), err.message());
+                           moduleID.first, err.message());
       }
 
       return nullptr;