Make VFS and FileManager match the current MemoryBuffer API.

This eliminates converting back and forth between the 3 formats and
gives us a more homogeneous interface.

llvm-svn: 220657

clang/unittests/Basic/VirtualFileSystemTest.cpp

@@ -39,14 +39,8 @@ public:
       return make_error_code(llvm::errc::no_such_file_or_directory);
     return I->second;
   }
-  std::error_code openFileForRead(const Twine &Path,
-                                  std::unique_ptr<vfs::File> &Result) override {
-    llvm_unreachable("unimplemented");
-  }
-  std::error_code getBufferForFile(const Twine &Name,
-                                   std::unique_ptr<MemoryBuffer> &Result,
-                                   int64_t FileSize = -1,
-                                   bool RequiresNullTerminator = true) {
+  ErrorOr<std::unique_ptr<vfs::File>>
+  openFileForRead(const Twine &Path) override {
     llvm_unreachable("unimplemented");
   }