[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365091 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Reader/BitcodeReader.cpp

@@ -20,7 +20,7 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/ADT/Twine.h"
-#include "llvm/Bitcode/BitstreamReader.h"
+#include "llvm/Bitstream/BitstreamReader.h"
 #include "llvm/Bitcode/LLVMBitCodes.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/IR/Argument.h"