Add new `sourceRootDir` setting

TypeScript Version: 3.5.1

Search Terms: sourceRoot, sources, sourcemap

Code

1. Create a module like: src/nested/foo.ts

2. Use the following tsconfig.json with tsc:

{
  "include": ["src"],
  "compilerOptions": {
    "baseUrl": ".",
    "outDir": "dist",
    "sourceMap": true,
    "sourceRoot": "src"
  }
}

Expected behavior:

Inside nested/foo.js.map, the sourceRoot should be ../src/ instead of src/. Otherwise, the paths in the sources array cannot be found.

Actual behavior:

The sourceRoot is src/ regardless of nesting.

Playground Link: N/A

Related Issues: None

Start by reproducing the issue with TypeScript 3.5.1 or typescript@next using the provided tsconfig.json and the nested src/nested/foo.ts module. Inspect the generated nested/foo.js.map and sourcemap generation path; done when a sourceRootDir setting produces a sourceRoot of ../src/ for nested output so the sources resolve correctly.