[Clang] Fix Hurd toolchain test on a two-stage build with ThinLTO

[Clang] Fix Hurd toolchain test on a two-stage build with ThinLTO

A two-stage ThinLTO build previously failed the clang/test/Driver/hurd.c test because of a static_cast in "tools::gnutools::Linker::ConstructJob()" which wrongly converted an instance of "clang::driver::toolchains::Hurd" into that of "clang::driver::toolchains::Linux". ThinLTO would later devirtualize the "ToolChain.getDynamicLinker(Args)" call and use "Linux::getDynamicLinker()" instead, causing the test to generate a wrong "-dynamic-linker" linker flag (/lib/ld-linux.so.2 instead of /lib/ld.so)

Fixes PR45061.

Differential Revision: https://reviews.llvm.org/D75373