RFR (S) 8149835: StringConcatFactory should emit classes with the same package as the host class

[Aleksey Shipilev]

Hi,

Please review a StringConcatFactory fix that makes generated String
concat stubs to be named "<host-class>$$StringConcat/<id>", instead of
"java/lang/String$$Concat/<id>":
  https://bugs.openjdk.java.net/browse/JDK-8149835

Among other things, this helps to dodge the failure caught by stricter
access controls in Jake (JDK-8149165), and hopefully caters for future
stricter Unsafe.defineAnonymousClass access controls (JDK-8058575).

Webrevs:
  http://cr.openjdk.java.net/~shade/8149835/webrev.jdk.01/
  http://cr.openjdk.java.net/~shade/8149835/webrev.langtools.01/

Testing: JPRT, java/lang/String; + copying SCF code to jake forest and
running with it.

Cheers,
-Aleksey