[foreign-jextract] RFR: 8248484: jextract should use raw names for generated identifiers as much as possible

[Athijegannathan Sundararajan]

jextracts avoid "C" prefix for struct/union/typedef nested classes.

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Commit messages:
 - 8248484: jextract should use raw names for generated identifiers as much as possible

Changes: https://git.openjdk.java.net/panama-foreign/pull/220/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/220/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248484
  Stats: 68 lines in 14 files changed: 1 ins; 0 del; 67 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/220.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/220/head:pull/220

PR: https://git.openjdk.java.net/panama-foreign/pull/220