libcxxabi [PR58117][NFC]: Open code lower bound

This open codes the use of lower-bound when looking for an operator
encoding. Using std::lower_bound can result in symbol references to
the C++ library and that breaks the ABI demangler, which mandates no
such dependency.

Differential Revision: https://reviews.llvm.org/D135799

Fixes: https://github.com/llvm/llvm-project/issues/58117