RFR: 7903758: jextract should avoid temporary file creation

[Athijegannathan Sundararajan]

jextract creates a temporary file to support multiple headers and special header syntax <foo.h>. This patch avoids that temporary file creation by using clang's in-memory parsing API clang_createTranslationUnitFromSourceFile

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Commit messages:
 - trailing whitespace removed
 - 7903758: jextract should avoid temporary file creation

Changes: https://git.openjdk.org/jextract/pull/251/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=251&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903758
  Stats: 3378 lines in 13 files changed: 2466 ins; 66 del; 846 mod
  Patch: https://git.openjdk.org/jextract/pull/251.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/251/head:pull/251

PR: https://git.openjdk.org/jextract/pull/251