Fix ExpandPrivate when compiling from TASTY

When compiling from TASTY, `ctx.source.file` will be the TASTY file
corresponding to the current compilation unit, this will never match
`d.symbol.sourceFile` whose value is set from the `SourceFile`
annotation in the TASTY tree. Fixed by using `ctx.owner.sourceFile`
instead.

The added test, `anonBridge.scala` needs this fix to work. It also needs
the previous work done in this PR to give positions to unpickled
symbols, otherwise it crashes in `Typer#assertPositioned` because
`Bridges#addBridge` creates a tree whose position comes from a symbol.

Author: smarter

compiler/src/dotty/tools/dotc/transform/ExpandPrivate.scala

@@ -90,8 +90,8 @@ class ExpandPrivate extends MiniPhase with IdentityDenotTransformer { thisPhase
       }
 
       assert(d.symbol.sourceFile != null &&
-             isSimilar(d.symbol.sourceFile.path, ctx.source.file.path),
-          s"private ${d.symbol.showLocated} in ${d.symbol.sourceFile} accessed from ${ctx.owner.showLocated} in ${ctx.source.file}")
+             isSimilar(d.symbol.sourceFile.path, ctx.owner.sourceFile.path),
+          s"private ${d.symbol.showLocated} in ${d.symbol.sourceFile} accessed from ${ctx.owner.showLocated} in ${ctx.owner.sourceFile}")
       d.ensureNotPrivate.installAfter(thisPhase)
     }
 

tests/pos-from-tasty/anonBridge.scala

@@ -0,0 +1,3 @@
+class Test {
+  val x: PartialFunction[Int, Int] = { case x: Int => x }
+}