[clang/mac] Accept -why_load and make -whyload an alias for it

[thakis]

Summary

From man ld:

-why_load   Log why each object file in a static library is loaded.
            That is, what symbol was needed.
            Also called -whyload for compatibility.

So -why_load is the spelling preferred by the linker and -whyload and old
compatibility setting. clang should accept the preferred form, and map both
forms to the preferred form.

[hans]

lgtm

nit in the change description

So -why_load is the spelling preferred by the linker and -whyload and old

second "and" -> "an"

clang/include/clang/Driver/Options.td

def v : Flag<["-"], "v">, Flags<[CC1Option, CoreOption]>,
   MarshallingInfoFlag<HeaderSearchOpts<"Verbose">>;
 def verify_debug_info : Flag<["--"], "verify-debug-info">, Flags<[NoXarchOption]>,
   HelpText<"Verify the binary representation of debug output">;
 def weak_l : Joined<["-"], "weak-l">, Flags<[LinkerInput]>;
 def weak__framework : Separate<["-"], "weak_framework">, Flags<[LinkerInput]>;
 def weak__library : Separate<["-"], "weak_library">, Flags<[LinkerInput]>;
 def weak__reference__mismatches : Separate<["-"], "weak_reference_mismatches">;
 def whatsloaded : Flag<["-"], "whatsloaded">;
-def whyload : Flag<["-"], "whyload">;
+def why_load : Flag<["-"], "why_load">;
+def whyload : Flag<["-"], "whyload">, Alias<why_load>;
 def w : Flag<["-"], "w">, HelpText<"Suppress all warnings">, Flags<[CC1Option]>,
   MarshallingInfoFlag<DiagnosticOpts<"IgnoreWarnings">>;
 def x : JoinedOrSeparate<["-"], "x">, Flags<[NoXarchOption,CC1Option]>,
   HelpText<"Treat subsequent input files as having type <language>">,
   MetaVarName<"<language>">;
 def y : Joined<["-"], "y">;
 
 defm integrated_as : BoolFOption<"integrated-as",

clang/lib/Driver/ToolChains/Darwin.cpp

void darwin::Linker::AddLinkArgs(Compilation &C, const ArgList &Args,
   Args.AddLastArg(CmdArgs, options::OPT_w);
   Args.AddAllArgs(CmdArgs, options::OPT_pagezero__size);
   Args.AddAllArgs(CmdArgs, options::OPT_segs__read__);
   Args.AddLastArg(CmdArgs, options::OPT_seglinkedit);
   Args.AddLastArg(CmdArgs, options::OPT_noseglinkedit);
   Args.AddAllArgs(CmdArgs, options::OPT_sectalign);
   Args.AddAllArgs(CmdArgs, options::OPT_sectobjectsymbols);
   Args.AddAllArgs(CmdArgs, options::OPT_segcreate);
-  Args.AddLastArg(CmdArgs, options::OPT_whyload);
+  Args.AddLastArg(CmdArgs, options::OPT_why_load);
   Args.AddLastArg(CmdArgs, options::OPT_whatsloaded);
   Args.AddAllArgs(CmdArgs, options::OPT_dylinker__install__name);
   Args.AddLastArg(CmdArgs, options::OPT_dylinker);
   Args.AddLastArg(CmdArgs, options::OPT_Mach);
 }
 
 /// Determine whether we are linking the ObjC runtime.
 static bool isObjCRuntimeLinked(const ArgList &Args) {