* [SwiftSyntax] Fix SyntaxChildren iteration
SyntaxChildren.Iterator stops the first time it sees a nil child;
since it traverses all indexes without considering presence, this
causes it to terminate iteration early if there is a missing child
in the list.
* [SwiftSyntax] Add tests for SyntaxChildren
* Unify the capitalization across all user-visible error messages (fatal errors, assertion failures, precondition failures) produced by the runtime, standard library and the compiler.
* Update some more tests to the new expectations.
* Create Swift libSyntax API
This patch is an initial implementation of the Swift libSyntax API. It
aims to provide all features of the C++ API but exposed to Swift.
It currently resides in SwiftExperimental and will likely exist in a
molten state for a while.
* Only build SwiftSyntax on macOS
