[llvm-lib] Ignore /LTCG option

"The /LTCG option to LIB specifies that the inputs from cl.exe include object
files generated by using the /GL compiler option."

Based on Microsoft's description
above (https://docs.microsoft.com/en-us/cpp/build/reference/running-lib?view=msvc-170),
there doesn't seem to be anything llvm-lib needs to do to support the flag.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D126000

llvm/lib/ToolDrivers/llvm-lib/Options.td

@@ -43,4 +43,5 @@ def help_q : Flag<["/??", "-??", "/?", "-?"], "">, Alias<help>;
 // The flags below do nothing. They are defined only for lib.exe compatibility.
 //==============================================================================
 
+def ltcg : F<"ltcg">;
 def nologo : F<"nologo">;