[C++][Gandiva] Add support for LLVM 11

[asfimport]

Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou

Related issues:

• [C++] Downgrade llvm to llvm@9 to correct build failures for gandiva (fixes)
• [C++][Gandiva] Add support for LLVM12 (is related to)

PRs and other links:

• GitHub Pull Request #8521

_{Note: This issue was originally created as ARROW-10385. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 8521
#8521