[foreign-jextract] RFR: 8262117: jextract crashes with javac compilation error "class u is already defined"

[Athijegannathan Sundararajan]

enclosing names are checked for clash now

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Commit messages:
 - 8262117: jextract crashes with javac compilation error "class u is already defined"

Changes: https://git.openjdk.java.net/panama-foreign/pull/458/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=458&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262117
  Stats: 110 lines in 4 files changed: 109 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/458.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/458/head:pull/458

PR: https://git.openjdk.java.net/panama-foreign/pull/458