Add a comment describing why libcalls are disabled for DXIL

Author: Icohedron

llvm/lib/Analysis/TargetLibraryInfo.cpp

@@ -205,6 +205,9 @@ static void initializeLibCalls(TargetLibraryInfoImpl &TLI, const Triple &T,
     return;
   }
 
+  // DXIL does not support libcalls, and disabling them here prevents a number
+  // of passes from introducing libcalls into DXIL which would otherwise
+  // complicate lowering/legalization
   if (T.isDXIL()) {
     TLI.disableAllFunctions();
     return;