[LLD] [MinGW] Ignore the --enable-auto-import flag

[mstorsjo]

Summary

In GNU ld, this option is enabled by default, but can be set to reduce some warnings.

For lld, ignore the flag (for now); in case linking still succeeds everything should be fine, if not, it should be clear to the user what part failed (possibly requiring adjusting the user project to not rely on this feature), instead of straight out failing due to an unknown flag.

[ruiu]

LGTM

lld/trunk/MinGW/Options.td

 def _HASH_HASH_HASH : Flag<["-"], "###">,
     HelpText<"Print (but do not run) the commands to run for this compilation">;
 
 // Currently stubs to avoid errors
 def Bdynamic: F<"Bdynamic">, HelpText<"Link against shared libraries">;
 def Bstatic: F<"Bstatic">, HelpText<"Do not link against shared libraries">;
 def O: Joined<["-"], "O">, HelpText<"Optimize output file size">;
 def enable_auto_image_base: F<"enable-auto-image-base">;
+def enable_auto_import: F<"enable-auto-import">;
 def full_shutdown: Flag<["--"], "full-shutdown">;
 def major_image_version: S<"major-image-version">;
 def minor_image_version: S<"minor-image-version">;
 def sysroot: J<"sysroot">, HelpText<"Sysroot">;
 def v: Flag<["-"], "v">, HelpText<"Display the version number">;
 def version: F<"version">, HelpText<"Display the version number and exit">;
 
 // Alias
 def alias_entry_e: JoinedOrSeparate<["-"], "e">, Alias<entry>;