[lld/COFF] Ignore /LTCG and /LTCG:FOO flags

[thakis]

Summary

We currently complain "could not open /LTCG: no such file or directory",
which isn't very useful. We could emit a warning when we see this flag, but
just ignoring it seems fine.

Final missing part of PR38799.

[aganea]

Sounds good.

lld/COFF/Options.td

291

Can you add /ilk: and /ltcgout: as well? This otherwise causes issues after VS 2019 16.9

[aganea]

lld/COFF/Options.td

291

Just to clarify: after VS 16.9, if these options aren't provided explicitly, the MSBuild property sheets would add these two options with default values.

[thakis]

lld/COFF/Options.td

291

Done, thanks :)

lld/COFF/Options.td

 def show_timing : F<"time">;
 def summary : F<"summary">;
 
 //==============================================================================
 // The flags below do nothing. They are defined only for link.exe compatibility.
 //==============================================================================
 
 def ignoreidl : F<"ignoreidl">;
+def ltcg : F<"ltcg">;
 def nologo : F<"nologo">;
 def throwingnew : F<"throwingnew">;
 def editandcontinue : F<"editandcontinue">;
 def fastfail : F<"fastfail">;
 
 def delay : P_priv<"delay">;
 def errorreport : P_priv<"errorreport">;
 def idlout : P_priv<"idlout">;
+def ilk : P_priv<"ilk">;
+def ltcg_opt : P_priv<"ltcg">;
+def ltcgout : P_priv<"ltcgout">;
 def maxilksize : P_priv<"maxilksize">;
 def tlbid : P_priv<"tlbid">;
 def tlbout : P_priv<"tlbout">;
 def verbose_all : P_priv<"verbose">;
 def guardsym : P_priv<"guardsym">;