Clang: Fix fallback compile features when simulating old MSVC versions

Refactoring in commit 25439c7d62 (Clang: Refactor CXX standard flags
into __compiler_clang_cxx_standards(), 2020-03-16, v3.18.0-rc1~362^2~4)
accidentally broke the `cxx_std_*` fallback feature names.

Issue: #21033
1 file changed