Fix <rdar://problem/22058555> QoI: unhelpful type error "cannot invoke 'withCString' with an argument list of type '((_) -> _)'"

The original issue has long since been fixed, but we were now producing:
error: cannot subscript a value of type 'UnsafePointer<Int8>'

which is pretty obviously wrong.  The problem is that when ranking subscript
decl candidates, CSDiags was using TC.isConvertibleTo to evaluate whether the
actual base type is compatible with the base type of a subscript decl.  This
was failing when the base was generic, because the logic isn't opening
archetypes.  Instead of incorrectly deciding that they are incompatible, just
decide we don't know if an archetype is present.  This allows us to generate
good errors in situation like this.

Author: lattner

lib/Sema/CSDiag.cpp

@@ -3334,6 +3334,8 @@ bool FailureDiagnosis::visitSubscriptExpr(SubscriptExpr *SE) {
     auto instanceTy =
      getter->getImplicitSelfDecl()->getType()->getInOutObjectType();
     if (!isUnresolvedOrTypeVarType(baseType) &&
+        // TODO: We're not handling archetypes well here.
+        !instanceTy->hasArchetype() &&
         !CS->TC.isConvertibleTo(baseType, instanceTy, CS->DC)) {
       selfConstraint = CC_SelfMismatch;
     }

test/Constraints/diagnostics.swift

@@ -638,14 +638,14 @@ _ = -UnaryOp() // expected-error {{unary operator '-' cannot be applied to an op
 // <rdar://problem/23433271> Swift compiler segfault in failure diagnosis
 func f23433271(x : UnsafePointer<Int>) {}
 func segfault23433271(a : UnsafeMutablePointer<Void>) {
-  f23433271(a[0])  // expected-error {{cannot subscript a value of type 'UnsafeMutablePointer<Void>' (aka 'UnsafeMutablePointer<()>')}}
+  f23433271(a[0])  // expected-error {{cannot convert value of type 'Void' (aka '()') to expected argument type 'UnsafePointer<Int>'}}
 }
 
 // <rdar://problem/22058555> crash in cs diags in withCString
 func r22058555() {
   var firstChar: UInt8 = 0
   "abc".withCString { chars in
-    firstChar = chars[0]  // expected-error {{cannot subscript a value of type 'UnsafePointer<Int8>'}}
+    firstChar = chars[0]  // expected-error {{cannot assign value of type 'Int8' to type 'UInt8'}}
   }
 }