Fix #7426: Set correct default source for unpickling

Note that we cannot use `classRoot.unforcedAnnotation(defn.SourceFileAnnot)`
because it's not loaded yet.

Nor can we use `SourceFile` in classfile attributes like the following:

      val sourceFile: SourceFile =
        if scan(tpnme.SourceFileATTR) then
          val attrLen = in.nextInt
          SourceFile.virtual(pool.getName(in.nextChar).toString, "")
        else NoSource

This is because `SourceFile` is classfile attributes only contains the file name,
while the `SourceFile` in TASTY contains the full path.

Author: liufengyun

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -8,7 +8,7 @@ import dotty.tools.tasty.{ TastyReader, TastyHeaderUnpickler }
 import Contexts._, Symbols._, Types._, Names._, StdNames._, NameOps._, Scopes._, Decorators._
 import SymDenotations._, unpickleScala2.Scala2Unpickler._, Constants._, Annotations._, util.Spans._
 import NameKinds.DefaultGetterName
-import ast.tpd._
+import ast.tpd._, util._
 import java.io.{ ByteArrayInputStream, ByteArrayOutputStream, DataInputStream, IOException }
 
 import java.lang.Integer.toHexString
@@ -750,7 +750,7 @@ class ClassfileParser(
 
       def unpickleTASTY(bytes: Array[Byte]): Some[Embedded]  = {
         val unpickler = new tasty.DottyUnpickler(bytes)
-        unpickler.enter(roots = Set(classRoot, moduleRoot, moduleRoot.sourceModule))
+        unpickler.enter(roots = Set(classRoot, moduleRoot, moduleRoot.sourceModule))(ctx.withSource(util.NoSource))
         Some(unpickler)
       }
 

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -63,6 +63,7 @@ class CompilationTests extends ParallelTesting {
       compileDir("tests/pos-special/adhoc-extension", defaultOptions.and("-strict", "-feature", "-Xfatal-warnings")),
       compileFile("tests/pos-special/i7575.scala", defaultOptions.and("-language:dynamics")),
       compileFile("tests/pos-special/kind-projector.scala", defaultOptions.and("-Ykind-projector")),
+      compileFile("tests/run/i5606.scala", defaultOptions.and("-Yretain-trees")),
     ).checkCompile()
   }