This splits the `--build-swift-stdlib` and `--build-swift-sdk-overlay`
arguments into `dynamic` and `static` variants, which makes the
following build command possible:
```
utils/build-script -- \
--build-swift-dynamic-stdlib=0 --build-swift-dynamic-sdk-overlay=0 \
--build-swift-static-stdlib=1 --build-swift-static-sdk-overlay=0
```
This command produces *only* static libraries for the stdlib, and no
SDK overlay libraries at all. Many other finely-grained build options
are now possible.
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