swift234

Fix <rdar://problem/22519983> QoI: Weird error when failing to infer archetype

Introduce a new "OpenedGeneric" locator for when openGeneric opens a generic
decl into a plethora of constraints, and use this in CSDiags to distinguish 
whether a constraint refers to an Expr as a whole or an "aspect" of the constraint.

Use that information in FailureDiagnosis::diagnoseGeneralConversionFailure
to know whether (as a fallback) we can correctly re-typecheck an entire expr 
to obtain a missing type.  If we are talking about an aspect of the expr, then
this clearly won't work.

The upshot of this is that where we previously compiled the testcase in 22519983
to:

y.swift:31:9: error: type '(inout _) -> Bool' does not conform to protocol 'RawRepresentable'
let a = safeAssign
        ^

we now produce the somewhat more useful:
y.swift:31:9: error: argument for generic parameter 'T' could not be inferred
let a = safeAssign
        ^
y.swift:27:6: note: in call to function 'safeAssign'
func safeAssign<T: RawRepresentable>(inout lhs: T) -> Bool {
     ^



Swift SVN r31620