Treat @_unavailableInEmbedded like other availability attributes during macro expansion.

[grynspan]

This PR adds support for mapping @_unavailableInEmbedded to a condition trait like we do with @available() etc.

Example:

@_unavailableInEmbedded
@Test func `stuff that doesn't work in Embedded Swift`() {
  ...
}

Which expands to include a trait of the form .__unavailableInEmbedded(sourceLocation: ...) of type ConditionTrait.

Checklist:

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