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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270604 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Reader/BitcodeReader.cpp

@@ -6382,6 +6382,9 @@ std::error_code ModuleSummaryIndexBitcodeReader::initLazyStream(
 }
 
 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 BitcodeErrorCategoryType : public std::error_category {
   const char *name() const LLVM_NOEXCEPT override {
     return "llvm.bitcode";