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.

lib/Serialization/SerializedModuleLoader.cpp

@@ -67,7 +67,7 @@ openModuleFiles(StringRef DirName, StringRef ModuleFilename,
   Scratch.clear();
   llvm::sys::path::append(Scratch, DirName, ModuleDocFilename);
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> ModuleDocOrErr =
-  llvm::MemoryBuffer::getFile(StringRef(Scratch.data(), Scratch.size()));
+      llvm::MemoryBuffer::getFile(StringRef(Scratch.data(), Scratch.size()));
   if (!ModuleDocOrErr &&
       ModuleDocOrErr.getError() != std::errc::no_such_file_or_directory) {
     return ModuleDocOrErr.getError();