When a `namespace` keyword has no `;` following it, we recover by
building a parse tree `Namespace` node from the `Namespace` token (as
there isn't a `;` token). Allow this correspondence on errors.
Also teach the diagnostics in this case to avoid the end-of-file token
as that's almost always going to be a less meaningful location. Instead,
we can point at the introducer which should at least be in the code that
led to the error.
Richard Smith
Jon Ross-Perkins
Chandler Carruth
josh11bandJon Ross-Perkins