#expect(throws:) expression should accept non-Void return type and describe returned values

[stmontgomery]

This changes #expect(throws:) and similar macros so that their closure parameters accept any type instead of Void specifically.

Motivation:

This makes testing trivial throwing expressions of non-Void types easier, by not requiring syntax to discard the return value. For example:

func mightThrowXOrReturnY() throws -> Y { ... }
#expect(throws: X.self) {
  try mightThrowXOrReturnY() // ⚠️ Result of call to 'mightThrowXOrReturnY()' is unused
}

Modifications:

• Change closure parameters of relevant macro-related declarations to use generic R type instead of Void.
• Change implementation of relevant __check... functions to include a description of return value.
• Add new unit tests.
• Update documentation to mention that return values are always discarded.

Result:

The changes in this PR mean the warning above will no longer be emitted by the compiler. These changes also allow printing a description of the return value if the expectation fails and the expression does not throw an error, providing a more detailed and actionable failure message.

[stmontgomery]

@swift-ci please test

[stmontgomery]

@swift-ci please test

[stmontgomery]

@swift-ci please test Linux