Adds 'lib' reference directives - #23893

This change adds support for a /// <reference lib="name" /> directive, allowing a file to explicitly include an existing built-in lib file.

• When a built-in lib is referenced via the lib reference directive, any no-default-lib reference directive in the lib file is ignored. This is so that a lib reference in an imported package won't cause your current project to lose its default lib if you haven't specified one.
• Built-in lib files are referenced in the same fashion as the "lib" compiler option in tsconfig.json (e.g. use lib="es2015" and not lib="lib.es2015.d.ts", etc.).

In the long term this can help polyfill/shim packages like core-js or es6-shim.

NOTE: Apparently we were very close to running out of heap memory when running gulp tests after running gulp clean as gulp is holding onto too much memory. Individually each task is fine, but in concert they add up to a large amount of uncollected memory. To address this, I've added a script to run some of our builds out-of-process. This is not an issue in the jake builds.

Fixes #15732.
Supersedes #15780