[C++] Downgrade llvm to llvm@9 to correct build failures for gandiva

[asfimport]

llvm@10 was removed from homebrew and the default was changed to 11 which caused build failures for gandiva.
Homebrew/homebrew-core#62798 (comment)

Downgrade llvm@9 as a temporary fix for build failures.

Reporter: Naman Udasi

Related issues:

• [C++][Gandiva] Add support for LLVM 11 (is fixed by)

_{Note: This issue was originally created as ARROW-10361. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
ARROW-10385 will fix this.