[Bitcode] Move Bitstream to a separate library

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.

Differential Revision: https://reviews.llvm.org/D63899

llvm-svn: 365091