Updates the CMake version for Swift and the Swift Benchmarks to 3.19.6.
Updates the docs to reflect this change. Does not modify the required version for building the stdlib.
The guide as written results in lots of repositories being placed as siblings to `swift-project`, instead of inside of it. This also results in the `build` directory being a sibling. Changing the clone to the `swift` directory fixes this.
CMake: fix build for Apple Silicon hosts
When building with `build-script` using these arguments
```
utils/build-script --skip-build-benchmarks
--skip-ios --skip-watchos --skip-tvos
--swift-darwin-supported-archs "arm64"
--sccache --release-debuginfo --test
```
the build fails with
```
ninja: error: 'stdlib/swift-test-stdlib-macosx-x86_64',
needed by 'stdlib/CMakeFiles/swift-test-stdlib', missing and no known rule to make it
```
I think that the "Getting Started" guide should avoid hardcoding `x86_64` arguments, and suggest using `$(uname -m)` instead. `SWIFT_PRIMARY_VARIANT_ARCH_default` could also get its value from `CMAKE_HOST_SYSTEM_PROCESSOR` in the root `CMakeLists.txt`.
Resolves SR-13943.
This improves upon the existing documentation to provide a clearer end-to-end
workflow for new contributors and people who wish to build the toolchain
locally but do not intend to submit patches.
We also provide more directions for systematically utilizing our existing
documentation.
