RFR: 8304031: Classfile API cannot encode Primitive Class as Condy

[liach]

Without this patch, the Classfile API tries to encode PrimitiveClassDesc as CONSTANT_Class_info and error in `toInternalName`.

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Commit messages:
 - 8304031: Classfile API cannot encode Primitive Class as Condy

Changes: https://git.openjdk.org/jdk/pull/12996/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12996&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304031
  Stats: 74 lines in 2 files changed: 73 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12996.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12996/head:pull/12996

PR: https://git.openjdk.org/jdk/pull/12996