swift234

Stop emitting most duplicated only-as-generic-constraint errors.

Set type repr's as invalid after diagnosing an unsupported protocol
to stop duplicate diagnoses.

There were two causes here. First, top-level variable
declarations were being checked once by the Decl checker, and then
again by the Stmt checker. (This caused SR-38.)

Second, the Stmt checker is called by an AST visitor itself, which
already calls it once per statement. Using the
UnsupportedProtocolVisitor here meant that each interior sub statement
would get visited multiple times. Added a setRecurseIntoSubstatements()
on the visitor, and set it to false for the Stmt checker. This keeps
from revisiting statements multiple times.