[Frontend] Add option to explicitly import Builtin

[Azoy]

This PR adds a new frontend option, -enable-builtin-module, that enables a module to explicitly import Builtin when needed. As part of this, the explicit import gets emitted into interface files so that we no longer implicitly import this module.

Motivation for this new flag includes the fact that standard library modules who are not the core sometimes need to access these builtins, but can only do so via -parse-stdlib who in itself bring along lots of other quirks that don't apply to these modules. The biggest is that the standard library is unconditionally built with -assert-config Debug even in release modes of the standard library. Modules using this flag and using stdlib APIs who have _debugPreconditions must incur the code size and branch of these preconditions even though they may not want them in opaque parts of their libraries. With this new flag, those modules can explicitly import Builtin and remove their dependence on -parse-stdlib allowing them to 1. implicitly get import Swift like normal Swift code and 2. be built as normal Swift code. It would be nice if one day we sunsetted -parse-stdlib and all of the unique flags that come along with it were explicit in the stdlib's build settings.

[Azoy]

@swift-ci please test

[Azoy]

@swift-ci please test macOS