Add FIXMEs to all derived classes of std::error_category.

This helps make clear that we're moving away from std::error_code.

Differential Revision: http://reviews.llvm.org/D20592

llvm-svn: 270604
diff --git a/llvm/lib/DebugInfo/PDB/GenericError.cpp b/llvm/lib/DebugInfo/PDB/GenericError.cpp
index 66b3a7a..34e1799 100644
--- a/llvm/lib/DebugInfo/PDB/GenericError.cpp
+++ b/llvm/lib/DebugInfo/PDB/GenericError.cpp
@@ -15,6 +15,9 @@
 using namespace llvm::pdb;
 
 namespace {
+// FIXME: This class is only here to support the transition to llvm::Error. It
+// will be removed once this transition is complete. Clients should prefer to
+// deal with the Error value directly, rather than converting to error_code.
 class GenericErrorCategory : public std::error_category {
 public:
   const char *name() const LLVM_NOEXCEPT override { return "llvm.pdb"; }