swift234

Make the "type of expression is ambiguous without more context" diagnostic more
specific when it fails, by printing a potentially partially resolved type for the
ambiguous expression in question, which it carries information.  This can at least
tell what the ambiguous parts of the resultant type *are* in some cases (e.g. in
the Constraints/array_literal.swift case).  That said, this diagnostic is still
admittedly not great.

This also exposes a couple of cases where we produce bogus diagnostics in general 
(expr/cast/as_coerce.swift).  The issue here is that these shouldn't be ambiguous 
at all, they are being misreported due to 22320758), which I'll fix separately.



Swift SVN r31292