build: migrate to CMake based build of swift-format

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This allows us to build swift-format with dynamic linking against the
toolchain build of the swift-syntax and swift-argument-parser packages.
Wire up the swift-markdown build and hoist the swift-format build prior
to sourcekit-lsp. This sets us up for supporting swift-format based
formatting in the LSP.

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CC: @QuietMisdreavus @ahoppen @allevato

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Please test with following PRs:
swiftlang/swift-cmark#69
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please build toolchain windows platform

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Please test with following PRs:
swiftlang/swift-cmark#69
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please test

[compnerd]

Please test with following PRs:
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please test

[compnerd]

Please test with following PRs:
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please test windows platform

[compnerd]

Please test with following PRs:
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please test windows platform

[compnerd]

Please test with following PRs:
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please smoke test Linux platform

[compnerd]

Please test with following PRs:
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please smoke test macOS platform

[compnerd]

Please test with following PRs:
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please build toolchain

[compnerd]

Please test with following PRs:
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please test Windows platform

[compnerd]

Please test with following PRs:
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please test macOS platform

[compnerd]

Please test with following PRs:
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please build toolchain Windows platform

[compnerd]

Please test with following PRs:
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please build toolchain macOS platform

[compnerd]

Please test with following PRs:
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please build toolchain Windows platform

[compnerd]

Please test with following PRs:
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please test

[compnerd]

Please test with following PRs:
swiftlang/swift-markdown#141
swiftlang/swift-format#677
swiftlang/swift-installer-scripts#271

@swift-ci please build toolchain

[compnerd]

Verified that the generated installer works, the installed toolchain passes a smoke test. Additionally, verified that swift format continues to function. As a side effect, the installer is now <500 MB again after reaching ~600MB.