RFR: Classfile signature bang support

[Chen Liang]

Support parsing ! signatures in ClassFile API.

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Commit messages:
 - Classfile signature bang support

Changes: https://git.openjdk.org/valhalla/pull/1933/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1933&range=00
  Stats: 248 lines in 4 files changed: 212 ins; 4 del; 32 mod
  Patch: https://git.openjdk.org/valhalla/pull/1933.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1933/head:pull/1933

PR: https://git.openjdk.org/valhalla/pull/1933