Add quiet flag (-q, --quiet) to LoggingOptions.

[clayellis]

This PR adds a quiet flag (-q, --quiet) to swift commands which suppresses all but error output.

Motivation:

Closes #4395. Executable targets often depend on the output of other executable targets. When an executable target is run with swift, there is currently no way to suppress the build output, which makes it more difficult to pass the output of one executable as the input of another. The new quiet flag suppresses all output except errors.

Modifications:

• Added a @Flag to LoggingOptions
• Interpret that flag as a Diagnostic.Severity in LoggingOptions.logLevel.
• Added tests for both the short and long flag.

Result:

When a user passes the quiet flag, only errors will be logged.

[clayellis]

@tomerd I followed your suggestions in making these changes. I decided not to introduce the log-level option and left it for someone else as it may be useful to express the existing flags in terms of the log-level option, which would be a larger change.

[MaxDesiatov]

@swift-ci smoke test

[tomerd]

thanks @clayellis