[MinGW] Include --enable-auto-import in the help listing

[mstorsjo]

Summary

Libtool concludes that the linker doesn't support shared libraries, unless this flag is listed in the output of --help.

[ruiu]

LGTM

configure and libtool were built for improving program portability, but since they are build based on hacky feature detection logic, they sometimes hurt program portability than improving it. It is an irony.

MinGW/Options.td

 def alias_strip_s: Flag<["-"], "s">, Alias<strip_all>;
 def alias_strip_S: Flag<["-"], "S">, Alias<strip_debug>;
 
 // Ignored options
 def: Joined<["-"], "O">;
 def: F<"build-id">;
 def: F<"disable-auto-image-base">;
 def: F<"enable-auto-image-base">;
-def: F<"enable-auto-import">;
+def: F<"enable-auto-import">, HelpText<"Ignored; listed for libtool compatibility">;
 def: F<"end-group">;
 def: Flag<["--"], "full-shutdown">;
 def: F<"high-entropy-va">;
 def: S<"major-image-version">;
 def: S<"minor-image-version">;
 def: F<"no-seh">;
 def: F<"nxcompat">;
 def: F<"pic-executable">;
 def: S<"plugin">;
 def: J<"plugin=">;
 def: S<"plugin-opt">;
 def: J<"plugin-opt=">;
 def: J<"sysroot">;
 def: F<"start-group">;
 def: F<"tsaware">;

test/MinGW/driver.test

 RUN: ld.lld -### -m i386pep foo.o -appcontainer | FileCheck -check-prefix APPCONTAINER %s
 RUN: ld.lld -### -m i386pep foo.o --appcontainer | FileCheck -check-prefix APPCONTAINER %s
 APPCONTAINER: -appcontainer
 
 # RUN: ld.lld -m i386pep --version 2>&1 | FileCheck -check-prefix=VERSION %s
 # RUN: ld.lld -m i386pep -v 2>&1 | FileCheck -check-prefix=VERSION %s
 # RUN: not ld.lld -m i386pep -v xyz 2>&1 | FileCheck -check-prefix=VERSION %s
 # VERSION: LLD {{.*}} (compatible with GNU linkers)
+
+# RUN: ld.lld -m i386pep --help 2>&1 | FileCheck -check-prefix=HELP %s
+# HELP: USAGE:
+# HELP: --enable-auto-import