Optimization: Don't keep FunProtos in wildcard approximations

FunProtos are uncached, so they should be approximated in cached implicit
computations.

Author: odersky

compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala

@@ -660,6 +660,8 @@ object ProtoTypes {
       tp.derivedViewProto(
           wildApprox(tp.argType, theMap, seen),
           wildApprox(tp.resultType, theMap, seen))
+    case tp: FunProto =>
+      WildcardType
     case  _: ThisType | _: BoundType => // default case, inlined for speed
       tp
     case _ =>