Require explicit backtrace and source location when creating and recording issues

[stmontgomery]

This refactors some of the logic for constructing SourceContext and Issue instances to require explicit arguments in more places.

Motivation:

We can provide clearer and more relevant details about recorded issues by ensuring that their backtrace and/or source location are accurate. In general, whenever collecting a backtrace, it's better to elide unhelpful frames from the testing library itself so that frames from user code are as close to the top of the call stack as possible. Similarly, whenever an explicit source location is known, it's best to propagate that in any recorded issues which originate at that location.

Currently, the SourceContext type has default parameter values for both its backtrace and sourceLocation properties. Having default values for these parameters means that callers elsewhere in the codebase may accidentally not specify a value, thereby not propagating useful information. Or, in the case of backtraces, they may rely on the default value which results in additional, irrelevant frames in the call stack (representing thunks inserted by the compiler for the default value). The initializer for Issue also includes a default value for one of its parameters.

Modifications:

• Change SourceContext.init(...) to remove the default values for both of its parameters: backtrace: and sourceLocation:.
• Change Issue.init(...) to no longer have default values for any of its parameters.
• Remove the non-public, static Issue.record(...) functions, and change callers to rely on the instance record(configuration:) method. This consolidates the usage pattern across the Testing module for creating an Issue and then recording it.
• Change SkipInfo.init(...) to remove the default value for its sourceContext: parameter.
• Add a convenience init(for:sourceLocation:) which accepts an Error and correctly creates a SourceContext for it.
• Add deprecated overloads of all SPIs which were modified to remove default parameter values.
• Add convenience overloads to TestingAdditions.swift which preserve default parameter values, to avoid unnecessary boilerplate in test-only code where it's safe to rely on such default values.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

This is internal code, so it doesn't make a huge difference, but… the intent behind not having a default value is to force us callers to always spell it out, lest we accidentally forget to forward comments from our own callers.

Is this fixing a known issue or just housekeeping?

[grynspan]

This change would introduce regressions in our backtrace quality and could cause us to record issues with no source context at all.

[stmontgomery]

@swift-ci please test