fix: getDefaultLibFilePath should normalize __dirname (microsoft#49051)

- this currently causes a bug on Windows with mixed path separators
  - it returns a POSIX backslash path for __dirname, and then adds a
    forward slash from the directorySeparator, causing mixed separators
    - TS uses `/` internally and lets the host convert if needed, so
      I assume this should be normalized to all forward slashses instead
  - example of the bug from my tests:
    ```
    Expected: "D:\\a\\rollup-plugin-typescript2\\rollup-plugin-typescript2\\node_modules\\typescript\\lib\\lib.d.ts"
    Received: "D:\\a\\rollup-plugin-typescript2\\rollup-plugin-typescript2\\node_modules\\typescript\\lib/lib.d.ts"
    ```

- every other use of __dirname in the codebase seems to be normalized
  except for this one
  - could use normalizeSlashes for this, but I figure combinePaths is
    more appropriate since that will handle it and combine properly as
    well without any `+ directorySeparator +` stuff

Author: agilgur5

src/services/services.ts

@@ -2880,7 +2880,7 @@ namespace ts {
     export function getDefaultLibFilePath(options: CompilerOptions): string {
         // Check __dirname is defined and that we are on a node.js system.
         if (typeof __dirname !== "undefined") {
-            return __dirname + directorySeparator + getDefaultLibFileName(options);
+            return combinePaths(__dirname, getDefaultLibFileName(options));
         }
 
         throw new Error("getDefaultLibFilePath is only supported when consumed as a node module. ");