Generate code coverage data for Xcode

[big-guy]

After generating the Xcode configuration for a project, enabling 'Gather coverage data' in the test scheme does nothing.

As a developer, I should be able to generate coverage data for my project, execute tests and visualize the results in Xcode.

Test coverage:

• When code coverage is enabled, Gradle should generate the appropriate code coverage files when running tests.
• When code coverage is not enabled, Gradle does not generate these files or compile test code with instrumentation options.
• Manual testing of Xcode checking for coverage data (Refer to: https://developer.apple.com/library/content/documentation/DeveloperTools/Conceptual/testing_with_xcode/chapters/07-code_coverage.html)

Out-of-scope:

• Code coverage with release builds.
• Producing a code coverage report from the command-line (unless this happens to be the same as Xcode needs)

[big-guy]

I'm satisfied that this has a potential workaround and would be low priority for now to remove it from the iteration.

I think this is just a matter of adding these as the compiler arguments to SwiftCompile -profile-generate -profile-coverage-mapping. We may want to model this as a isProfilable option on the task/binary.

We could key off of an environment variable (CLANG_COVERAGE_MAPPING) that Xcode sets to enable code profiling, but I think we should just add a new Xcode configuration that you need to switch to if you want to generate profile data. We would want to also consider adding another buildType on the Gradle side (e.g., debug, release, profile) so we could potentially generate code coverage reports ourselves.

As a workaround, it looks viable to just add those compiler arguments to the task and see how close that gets us. We would need to spend some time in Xcode to figure out if we need to do anything else.