Fix #10082: show friendly error message related to type inference

The type `site` might be an uninitialized type variable,
but is initialized when we display the error message.
It results in confusing error message as in #10082.

We take a string representation of the type `site`
when the error object is created and use that in the
message message.

Now the error message is more sensible:

-- [E008] Not Found Error: tests/neg/i10082.scala:7:30 -------------------------
7 |def Test = List(1).map(fun(it + 1))
  |                           ^^^^
  |                 value + is not a member of T
  |
  |                 where:    T is a type variable with constraint >: Int
  |                 , but could be made available as an extension method.

Author: liufengyun

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -286,6 +286,8 @@ import transform.SymUtils._
   extends NotFoundMsg(NotAMemberID) {
     //println(i"site = $site, decls = ${site.decls}, source = ${site.typeSymbol.sourceFile}") //DEBUG
 
+    val widenedName: String  = ex"${site.widen}"
+
     def msg = {
       import core.Flags._
       val maxDist = 3  // maximal number of differences to be considered for a hint
@@ -348,7 +350,7 @@ import transform.SymUtils._
             s" - did you mean $siteName.$n?$enumClause"
           case Nil => prefixEnumClause("")
 
-      ex"$selected $name is not a member of ${site.widen}$finalAddendum"
+      ex"$selected $name is not a member of $widenedName$finalAddendum"
     }
 
     def explain = ""