Add ConditionTrait.evaluate()

[Uncommon]

Add ConditionTrait.evaluate() so that a condition can be evaluated independent of a Test object.

Motivation:

Currently, the only way a ConditionTrait is evaluated is inside the prepare(for:) method. This makes it difficult and awkward for third-party libraries to utilize these traits because evaluating a condition would require creating a dummy Test to pass to that method.

Modifications:

Add ConditionTrait.evaluate(), and ConditionTrait.Evaluation enum for the return value.

Result:

Public API allows for evaluating a ConditionTrait in any context.

Checklist:

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

[Uncommon]

I'm trying to decide what tests to add. Are existing tests enough, since I pretty much just moved existing logic to a different place?

[grynspan]

We aim for 100% code coverage when testing, so the new function should be tested (even if it is just a trivial "did this return true when I expected it to?" test.)

[Uncommon]

I plan to add some tests soon, at which point I think it will be appropriate to take the PR out of draft mode.

[stmontgomery]

Thanks @Uncommon! Just a couple more comments but this looks close to landing. We can land this before the associated Swift evolution proposal review begins (assuming it moves forward), since the additions are @_spi.

[stmontgomery]

I meant to use "Request Changes" in the comment above.

[stmontgomery]

@swift-ci please test

[stmontgomery]

Fixes #903
Fixes rdar://142690932