build: setup a CMake based build for DocC

[compnerd]

In local testing, this helps reduce the overall build times 7-8%. Stage the build conversion to allow us to measure the time savings on CI hosts.

[compnerd]

Please test with following PRs:
apple/swift-driver#1540
apple/swift-lmdb#6
apple/swift-docc-symbolkit#70
apple/swift-docc#818

@swift-ci please build toolchain Windows platform

[compnerd]

Please test with following PRs:
apple/swift-driver#1540
apple/swift-lmdb#6
apple/swift-docc-symbolkit#70
apple/swift-docc#818

@swift-ci please build toolchain Windows platform

[compnerd]

Please test with following PRs:
apple/swift-driver#1540
apple/swift-lmdb#6
apple/swift-docc-symbolkit#70
apple/swift-docc#818

@swift-ci please build toolchain Windows platform

[compnerd]

Please test with following PRs:
apple/swift-driver#1540
apple/swift-lmdb#6
apple/swift-docc-symbolkit#70
apple/swift-docc#818

@swift-ci please build toolchain Windows platform

[compnerd]

Please test with following PRs:
apple/swift-lmdb#6
apple/swift-docc-symbolkit#70
apple/swift-docc#818

@swift-ci please build toolchain Windows platform

[compnerd]

Please test with following PRs:
apple/swift-lmdb#6
apple/swift-docc-symbolkit#70
apple/swift-docc#818
apple/swift-installer-scripts#277

@swift-ci please build toolchain Windows platform

[compnerd]

Please test with following PRs:
apple/swift-lmdb#6
apple/swift-docc-symbolkit#70
apple/swift-docc#818
apple/swift-installer-scripts#277

@swift-ci please test