RFR: 7903758: jextract can avoid temporary file creation for single simple header case

[Athijegannathan Sundararajan]

jextract creates a temporary file to support multiple headers and special header syntax <foo.h>. But, temporary file creation can be avoided for single, simple header file/path case.

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Commit messages:
 - 7903758: jextract can avoid temporary file creation for single simple header case

Changes: https://git.openjdk.org/jextract/pull/250/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=250&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903758
  Stats: 33 lines in 2 files changed: 25 ins; 5 del; 3 mod
  Patch: https://git.openjdk.org/jextract/pull/250.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/250/head:pull/250

PR: https://git.openjdk.org/jextract/pull/250