RFR: 8343064: ClassFormatError: Illegal class name from InnerClassLambdaMetafactory

[Chen Liang]

After the ClassFile API migration, when serializable lambdas are requested for hidden class callers, illegal class name is generated for the serialization methods, which previously had legal yet unusable class names, as hidden classes cannot be described by a `CONSTANT_Class_info`.

This patch restores a similar older behavior of using legal yet unusable class names.  Previously, the invalid `.` in hidden class names was replaced by a `/` as if a package separator; this patch changes it to a `_` like that in the generated lambda's class name.

The bug report shows some unintended and erroneous usages of `LambdaMetafactory`.  To clarify and to persuade against misuse, I added a paragraph of API notes to `LambdaMetafactory`, describing the impact of this API being designed to support Java language features.  In particular, I used the scenario where people assumed LMf generates weak hidden classes, which happened in this issue report and in #12493, that misuse can lead to resource leaks.

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Commit messages:
 - Specify consequence
 - 8343064: ClassFormatError: Illegal class name from InnerClassLambdaMetafactory

Changes: https://git.openjdk.org/jdk/pull/21912/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21912&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343064
  Stats: 21 lines in 2 files changed: 17 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21912.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21912/head:pull/21912

PR: https://git.openjdk.org/jdk/pull/21912