LLVM trunk commit a5479197371e169bc676e0e570f2a994329f8773 split include/llvm/Bitcode/ReaderWriter.h into two separate header files.  This CL hacks it back, in a sense.

With this change (along with the rest of the recent fixes), Subzero builds against the latest LLVM trunk - 759dd39154f0bbf1adc87bf613c95f9564f64aa8 at this writing.

BUG= none
R=kschimpf@google.com

Review-Url: https://codereview.chromium.org/2604753002 .