[flang] Fix clang build on MacOS

clang doesn't consider `std::strlen` of a constant to be a constexpr,
so just replace with the actual length.

Original-commit: flang-compiler/f18@738deab4656478df41a3ef7fdb9f02fe503779c5
Reviewed-on: https://github.com/flang-compiler/f18/pull/761