devel/opencl-clang: unbreak llvm15 flavor on FreeBSD 12.3

FAILED: opencl_clang_options.inc ../.build/opencl_clang_options.inc
cd ../.build && /usr/bin/llvm-tblgen -gen-opt-parser-defs -I /usr/local/llvm15/include -I . opencl_clang_options.td --write-if-changed -o opencl_clang_options.inc -d opencl_clang_options.inc.d
Included from opencl_clang_options.td:8:
/usr/local/llvm15/include/llvm/Option/OptParser.td:16:66: error: Variable not defined: 'false'
class OptionKind<string name, int precedence = 0, bit sentinel = false> {
                                                                 ^
Included from opencl_clang_options.td:8:
/usr/local/llvm15/include/llvm/Option/OptParser.td:25:18: error: Value not specified for template argument #2 (OptionKind:sentinel) of subclass 'OptionKind'!
def KIND_GROUP : OptionKind<"Group">;
                 ^