[C++][Gandiva] Add support for LLVM12

[asfimport]

MacOS CI builds have started failing recently as Homebrew has updated LLVM to version 12. 

Reporter: David Li / @lidavidm
Assignee: Krisztian Szucs / @kszucs

Related issues:

• [CI][C++][Gandiva] MacOS builds are failing because of the version of the LLVM (duplicates)
• [C++] Support LLVM 12 (is duplicated by)
• [C++][Gandiva] Add support for LLVM 11 (relates to)

PRs and other links:

• GitHub Pull Request #10111

_{Note: This issue was originally created as ARROW-12467. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 10111
#10111