[clang-tidy] Don't try to build CTTestTidyModule for Windows with dylibs

In MinGW mode, it's possible to build LLVM/Clang with
LLVM_LINK_LLVM_DYLIB (which implicitly enables plugins too). Other
existing ways of building plugins on Windows is to build with
LLVM_EXPORT_SYMBOLS_FOR_PLUGINS, where each executable exports its
symbols.

With LLVM_LINK_LLVM_DYLIB, we can't generally skip building plugins
even if they are set up with PLUGIN_TOOL, as some plugins (e.g.
under clang/examples) set up that way do build properly (as
they manually call clang_target_link_libraries, which links in the
libclang-cpp.dll dylib).

For CTTestTidyModule, there's no corresponding dylib that would
provide the same exports.

Differential Revision: https://reviews.llvm.org/D121687