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ARROW-11245: [C++][Gandiva] Add support for LLVM 11.1

It's not released yet but RC1 is available.

Closes #9200 from kou/cpp-llvm-11.1

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>