[lld/mac] Stop accepting arbitrary suffixes on --(no-)warn-dylib-install-name

The flag accidentally used Joined<> instead of Flag<>.

Previously, `--warn-dylib-install-namefoobarbaz` would be accepted and
had the same effect as `-warn-dylib-install-name`. Now the flag only
works if no suffix is attached to it, as originally intended.

Also fix a typo in the flag's help text.

Differential Revision: https://reviews.llvm.org/D131781

Author: nico

lld/MachO/Options.td

@@ -77,11 +77,11 @@ def start_lib: Flag<["--"], "start-lib">,
     HelpText<"Start a grouping of objects that should be treated as if they were together in an archive">;
 def end_lib: Flag<["--"], "end-lib">,
     HelpText<"End a grouping of objects that should be treated as if they were together in an archive">;
-def no_warn_dylib_install_name: Joined<["--"], "no-warn-dylib-install-name">,
-    HelpText<"Do not warn on -install-name if -dylib is not passed (default)">,
+def no_warn_dylib_install_name: Flag<["--"], "no-warn-dylib-install-name">,
+    HelpText<"Do not warn on -install_name if -dylib is not passed (default)">,
     Group<grp_lld>;
-def warn_dylib_install_name: Joined<["--"], "warn-dylib-install-name">,
-    HelpText<"Warn on -install-name if -dylib is not passed">,
+def warn_dylib_install_name: Flag<["--"], "warn-dylib-install-name">,
+    HelpText<"Warn on -install_name if -dylib is not passed">,
     Group<grp_lld>;
 def call_graph_profile_sort: Flag<["--"], "call-graph-profile-sort">,
     HelpText<"Reorder sections with call graph profile (default)">,