swift234

Introduce a new TypeCheckExprFlags::AllowUnresolvedTypeVariables option,
which allows solving of a constraint system to succeed without emitting
errors in the face of ambiguous solutions.  This is important for CSDiag
because it is in the business of trying to solve subexpressions of a global
expression - and it wants to know the difference between a subexpression
that is inherently impossible to solve, vs one that is simply ambiguous
because its context has been removed.

Use this in CSDiag's typeCheckChildIndependently() to provide it an
extra flag that enables this behavior.  This is currently unused, so NFC 
with this patch.


Swift SVN r30402