[jdk11u-dev] RFR: 8305721: add `make compile-commands` artifacts to .gitignore

[Joshua Cao]

Not a clean backport due since JDK11 does not have .gitignore changes from [8295884](https://bugs.openjdk.org/browse/JDK-8295884)

-------------

Commit messages:
 - 8305721: add `make compile-commands` artifacts to .gitignore

Changes: https://git.openjdk.org/jdk11u-dev/pull/1827/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1827&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305721
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1827.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/1827/head:pull/1827

PR: https://git.openjdk.org/jdk11u-dev/pull/1827