formula.rb: Change fails_with_llvm for XCode 4.2
formula.rb: Change fails_with_llvm for XCode 4.2

Suggest trying `--use-clang` instead of `--use-gcc` (which does nothing).

Signed-off-by: Adam Vandenberg <34c2b6407fd5a10249a15d699d40f9ed1782e98c@gmail.com>

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