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

[Erik Joelsson]

On Thu, 6 Apr 2023 23:59:55 GMT, Joshua Cao <duke at openjdk.org> wrote:

> `make compile-commands` produces
> 
> * `compile-commands.json` - so clangd knows how to build the project
> * `.cache` - I believe clangd uses this as an indexing cache
> 
> Tested by confirming artifacts are not shown when running `git status`.

Marked as reviewed by erikj (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/13381#pullrequestreview-1376170508