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

[sbc100]

Summary

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

llvm/include/llvm/Support/CodeGen.h

 
 #ifndef LLVM_SUPPORT_CODEGEN_H
 #define LLVM_SUPPORT_CODEGEN_H
 
 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 };
   }
 
   // Code model types.
   namespace CodeModel {
     // Sync changes with CodeGenCWrappers.h.
     enum Model { Tiny, Small, Kernel, Medium, Large };
   }
 
   namespace PICLevel {
     // This is used to map -fpic/-fPIC.
     enum Level { NotPIC=0, SmallPIC=1, BigPIC=2 };
   }
 
   namespace PIELevel {