FrontendInputs data structure redo.

- Outlaw duplicate input files, fix driver, fix tests, and add test.
- Reflect that no buffer is present without a (possibly pseudo) named file.
- Reflect fact that every input has a (possible pseudo) name.
- Break up CompilerInstance::setup.

Don't bail on dups.

tools/SourceKit/lib/SwiftLang/SwiftIndexing.cpp

@@ -279,7 +279,7 @@ void SwiftLangSupport::indexSource(StringRef InputFile,
     return;
   }
 
-  if (!Invocation.getFrontendOptions().Inputs.hasInputFilenames()) {
+  if (!Invocation.getFrontendOptions().Inputs.hasInputs()) {
     IdxConsumer.failed("no input filenames specified");
     return;
   }