[CSSyntacticElement] Make sure that body of tap is solved in isolation

It doesn't need access to anything besides external declarations
which could be brought into the scope by context analyzer.

Author: xedin

lib/Sema/CSSyntacticElement.cpp

@@ -320,6 +320,11 @@ static void createConjunction(ConstraintSystem &cs,
     if (auto *closure = dyn_cast<ClosureExpr>(tapDC)) {
       referencedVars.push_back(cs.getType(closure)->castTo<TypeVariableType>());
     }
+
+    // Body of the interpolation is always isolated from its context, only
+    // its individual elements are allowed access to type information
+    // from the outside e.g. external declaration references.
+    isIsolated = true;
   }
 
   UnresolvedClosureParameterCollector paramCollector(cs);