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/swift-syntax-test/swift-syntax-test.cpp

@@ -135,7 +135,7 @@ SourceFile *getSourceFile(CompilerInstance &Instance,
                           const char *MainExecutablePath) {
   CompilerInvocation Invocation;
   Invocation.getLangOptions().KeepSyntaxInfoInSourceFile = true;
-  Invocation.addInputFilename(InputFileName);
+  Invocation.getFrontendOptions().Inputs.addInputFile(InputFileName);
   Invocation.setMainExecutablePath(
     llvm::sys::fs::getMainExecutable(MainExecutablePath,
       reinterpret_cast<void *>(&anchorForGetMainExecutable)));