interface: pass correct 4th argument to CompilerInvocation::setLangDefaults

Since llvmorg-12-init-16745-gc495dfe0268b ([clang][cli] NFC: Decrease
the scope of ParseLangArgs parameters, Thu Jan 14 08:26:12 2021 +0100),
the fourth argument of CompilerInvocation::setLangDefaults is
a std::vector<std::string> rather than a clang::PreprocessorOptions.
Introduce a setLangDefaultsArg4 to pass in the right argument
depending on what is expected by the specific version of clang.

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>