[CS] Fix an over-eager assert

We can in fact have placeholders here if the type is bound to a hole.

Author: hamishknight

lib/Sema/CSSyntacticElement.cpp

@@ -803,8 +803,7 @@ class SyntacticElementConstraintGenerator
       auto projectedTy = computeProjectedValueType(wrappedVar, wrapperTy);
       // The projected type may have an error, make sure we turn it into a hole
       // if necessary.
-      ASSERT(!projectedTy->hasUnboundGenericType() &&
-             !projectedTy->hasPlaceholder());
+      ASSERT(!projectedTy->hasUnboundGenericType());
       projectedTy = cs.replaceInferableTypesWithTypeVars(projectedTy, locator);
       cs.setType(projectedVal, projectedTy);
     }

validation-test/compiler_crashers_fixed/SyntacticElementConstraintGenerator-visitDecl-8830af.swift

@@ -0,0 +1,11 @@
+// {"kind":"typecheck","original":"25782df5","signature":"(anonymous namespace)::SyntacticElementConstraintGenerator::visitDecl(swift::Decl*)","signatureAssert":"Assertion failed: (!projectedTy->hasUnboundGenericType() && !projectedTy->hasPlaceholder()), function setPropertyWrapperAuxiliaryTypes"}
+// RUN: not %target-swift-frontend -typecheck %s
+@propertyWrapper
+struct a<
+  b
+> {
+  projectedValue : Self  var wrappedValue: b
+}
+{
+  @a var c: <#type#>
+}