[LLD] [MinGW] Ignore the -rpath option

GNU ld silently accepts the -rpath option for Windows targets,
as a no-op.

This has lead to some build systems (and users) passing this
option while building for Windows/MinGW, even if Windows doesn't
have any concept like rpath.

Older versions of Conan did include -rpath in the pkg-config
files it generated, see e.g.
https://github.com/conan-io/conan/blob/17c58f0c61931f9de218ac571cd97a8e0befa68e/conans/client/generators/pkg_config.py#L104-L114
and
https://github.com/conan-io/conan/blob/17c58f0c61931f9de218ac571cd97a8e0befa68e/conans/client/build/compiler_flags.py#L26-L34
- and see mstorsjo/llvm-mingw#300
for user reports about this issue.

Silently ignore the option in LLD for MinGW targets, to improve
drop-in compatibility compared to GNU ld.

Author: mstorsjo

lld/MinGW/Options.td

@@ -239,6 +239,7 @@ defm: EqNoHelp<"minor-image-version">;
 def: F<"no-undefined">;
 def: F<"pic-executable">;
 defm: EqNoHelp<"plugin">;
+defm: EqNoHelp<"rpath">;
 defm: EqNoHelp<"sysroot">;
 def: F<"sort-common">;
 def: F<"start-group">;