swift234

Fix <rdar://problem/24267414> QoI: error: cannot convert value of type 'Int' to specified type 'Int'

When a contextual conversion has a matching type, don't diagnose it as a
conversion error, the problem is due to something else (in this case, an
unresolved archetype somewhere else in the expression).

Before:
t.swift:6:17: error: cannot convert value of type 'Int' to specified type 'Int'
After:
t.swift:6:17: error: generic parameter 'T' could not be inferred

This should still be a bit better to provide information about where the T
archetype came from, but at least now it isn't completely wrong diagnostic.