swift234

[Sema] Remove unnecessary check for potentially unavailable inferred type parameter actuals

This commit removes an unnecessary check that ensured that inferred type
parameter actuals are available. This check is unnecessary because the
declaration for any inferred actual type parameter (i.e., a generic type
argument at the callsite) must be referred to in an enclosing type refinement
context. If the inferred type is not available, this reference will cause a
diagnostic to be emitted, so we don't need to emit another diagnostic for the
inferred actual. Further, this diagnostic was potentially confusing because the
point at which we emitted the diagnostic did not directly mention the type
name (because it was inferred).

This commit also beefs up testing of diagnostics for potentially unavailable
type parameters.

Swift SVN r25540