Restore comment regarding why Reloc::PIC_ can't be PIC

The original change back in rL29307 explained this but it was
lost somewhere along the way.

Differential Revision: https://reviews.llvm.org/D59445

llvm-svn: 356402

llvm/include/llvm/Support/CodeGen.h

@@ -18,6 +18,7 @@ namespace llvm {
 
   // Relocation model types.
   namespace Reloc {
+    // Cannot be named PIC due to collision with -DPIC
     enum Model { Static, PIC_, DynamicNoPIC, ROPI, RWPI, ROPI_RWPI };
   }