RFR: JDK-8301460: Clean up LambdaForm to reference BasicType enums directly

[Mandy Chung]

`LambdaForm` declares int constants for `BasicType::ordinal` to workaround JDK-8161245.   Now these int constants are no longer needed.    This removes these int constants and reference `BasicType` enums directly.

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Commit messages:
 - JDK-8301460: Code in LambdaForm.java still refers to resolved JDK-8161245

Changes: https://git.openjdk.org/jdk/pull/12546/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12546&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301460
  Stats: 29 lines in 3 files changed: 9 ins; 10 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/12546.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12546/head:pull/12546

PR: https://git.openjdk.org/jdk/pull/12546