This moves Bitcode/Bitstream*, Bitcode/BitCodes.h to Bitstream/.
This is needed to avoid a circular dependency when using the bitstream code for parsing optimization remarks.
Since Bitcode uses Core for the IR part:
libLLVMRemarks -> Bitcode -> Core
and Core uses libLLVMRemarks to generate remarks (see IR/RemarkStreamer.cpp):
Core -> libLLVMRemarks
we need to separate the Bitstream and Bitcode part.
For clang-doc, it seems that it doesn't need the whole bitcode layer, so I updated the CMake to only use the bitstream part.