swift234

Be more tolerant of invalid protocol conformances when applying constraint solution.

* Lift NormalProtocolConformance::hasTypeWitness() to
  ProtocolConformance. This method can be used to determine whether a
  potentially invalid conformance has a particular type witness.

* ProtocolConformance::getWitness() may return nullptr if the witness
  does not exist.

* In TypeChecker::getWitnessType(), don't complain that a builtin
  protocol is broken if it's the conformance that's broken.

* ConformanceChecker should only emit diagnostics from within calls to
  checkConformance().

* TypeChecker::conformsToProtocol() now returns true if the type
  has a declared conformance to the protocol, regardless of whether the
  conformance is valid. Clients must check the validity of the
  ProtocolConformance themselves if necessary.

<rdar://problem/19495341> Can't upcast to parent types of type constraints without forcing

Swift SVN r25326