Merge pull request swiftlang#243 from keith/ks/multithread-processing…

…-source-files

Multithread processing source files

Merge pull request swiftlang#236 from allevato/swift-5.3-version

Update version number printed by `-v` flag.

Merge pull request swiftlang#194 from allevato/5.2-cherrypicks

Cherry-pick recent 5.2-compatible changes into the 5.2 branch.

Merge pull request swiftlang#171 from allevato/swift-5.2-branch

Cherrypick recent changes into swift-5.2-branch.