[lworld] RFR: JDK-8252061 [lword] Support Object::toString for inline type

[Mandy Chung]

This adds the `Object::toString` support for inline classes.  This will replace the need
for javac to generate the `toString` method to call the bootstrap method.

JDK-8206077 will revisit the overlap of `ValueBootstrapMethods` and `ObjectMethods`.

-------------

Commit messages:
 - update javadoc of Object::toString
 - fix ValueModifierTest for this change'
 - Merge branch 'lworld' of https://github.com/openjdk/valhalla into substitutability
 - [lworld] Object::toString support for inline type

Changes: https://git.openjdk.java.net/valhalla/pull/155/files
 Webrev: https://webrevs.openjdk.java.net/valhalla/155/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252061
  Stats: 451 lines in 12 files changed: 150 ins; 264 del; 37 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/155.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/155/head:pull/155

PR: https://git.openjdk.java.net/valhalla/pull/155