Fix isInlineable check to work during overloading resolution

Overloading may create temporary symbols via `Applications#resolveMapped`, these
symbols do not carry the annotations from the original symbols which means the
`isInlineable` would always return false for them. This matters because during
the course of overloading resolution we might call
`ProtoTypes.Compatibility#constrainResult` which special-cases transparent
inline methods.

Fixes a regression in Monocle introduced in the previous commit.

compiler/src/dotty/tools/dotc/inlines/Inlines.scala

@@ -49,7 +49,13 @@ object Inlines:
 
   /** Can a call to method `meth` be inlined? */
   def isInlineable(meth: Symbol)(using Context): Boolean =
-    meth.is(Inline) && meth.hasAnnotation(defn.BodyAnnot) && !inInlineMethod
+    meth.is(Inline)
+    && (meth.hasAnnotation(defn.BodyAnnot)
+        // Ensure `isInlineable` works with temporary symbols created during
+        // overloading resolution by `Applications#resolveMapped`.
+        // Testcase: tests/pos/i21410c.scala
+        || meth.hasAnnotation(defn.MappedAlternativeAnnot))
+    && !inInlineMethod
 
   /** Should call be inlined in this context? */
   def needsInlining(tree: Tree)(using Context): Boolean = tree match {

tests/pos/i21410c.scala

@@ -0,0 +1,11 @@
+class AppliedPIso[A, B]()
+case class User(age: Int)
+
+object Test:
+  extension [From, To](from: From)
+    def focus(): AppliedPIso[From, From] = ???
+    transparent inline def focus(inline lambda: (From => To)): Any = ???
+
+
+  val u = User(1)
+  val ap: AppliedPIso[User, User] = u.focus(_.age) // error