Merge pull request #10552 from Bo98/sdk-checks

Library/Homebrew/extend/os/mac/development_tools.rb

Library/Homebrew/extend/os/mac/diagnostic.rb

Library/Homebrew/os/mac/sdk.rb

Library/Homebrew/os/mac/version.rb

Library/Homebrew/os/mac/xcode.rb

Library/Homebrew/test/os/mac/diagnostic_spec.rb

Library/Homebrew/test/os/mac/sdk_spec.rb

...support/fixtures/sdks/big_sur/MacOSX.sdk/SDKSettings.json

...xtures/sdks/high_sierra/MacOSX10.13.sdk/SDKSettings.plist

.../fixtures/sdks/malformed/MacOSX10.15.sdk/SDKSettings.json

...tures/sdks/mojave_broken/MacOSX10.14.sdk/SDKSettings.json

Library/Homebrew/version.rb