Get .tasty files from the correct jar

1. If we don't give a parent to the URLClassLoader constructor, it will use
the system ClassLoader by default, which includes the jars in the JVM classpath.
2. `getResourceAsStream` will first look for the resource in a parent
ClassLoader.
3. The reference compiler is in the JVM classpath.

So if the reference compiler is Dotty-compiled, we might pick up .tasty
files from it instead of the current compiler. Using `null` as the
parent ClassLoader avoids this problem.

Author: smarter

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

@@ -804,7 +804,7 @@ class ClassfileParser(
               ctx.error("Could not load TASTY from .tasty for virtual file " + classfile)
               Array.empty
             case Some(jar: ZipArchive) => // We are in a jar
-              val cl = new URLClassLoader(Array(jar.jpath.toUri.toURL))
+              val cl = new URLClassLoader(Array(jar.jpath.toUri.toURL), /*parent =*/ null)
               val path = classfile.path.stripSuffix(".class") + ".tasty"
               val stream = cl.getResourceAsStream(path)
               if (stream != null) {