[LLD][COFF] Ignore /pdbcompress flag

[pzheng]

Summary

Microsoft does not seem to document the flag. Ignoring it for now is probably
better than getting an unknown flag error.

[thakis]

lgtm.

lld/COFF/Options.td

 //==============================================================================
 
 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 pdbcompress : F<"pdbcompress">;
 
 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">;