API: incorrect declarations in types.ts

The `--strictNullChecks` PR changed some declarations in `types.ts` to avoid frequent assertions in the compiler's code. Unfortunately these changes make it really unsafe for API users because they no longer contain the actually nullable types: https://github.com/Microsoft/TypeScript/pull/22088#discussion_r184149465 https://github.com/Microsoft/TypeScript/pull/22088#discussion_r184155027 * `ts.Node#parent` is no longer optional (which is definitely wrong for `SourceFile`) * `ts.Symbol#declarations` and `ts.Symbol.valueDeclaration` are no longer optional, but can still be undefined * `ts.Type#symbol` is no longer optional, but I guess this comment is still up to date: `// Symbol associated with type (if any)` These changes should either be reverted or replaced in the published declaration files. /cc @andy-ms @weswigham