Mirror the llvm changes that split Bitcode/ReaderWriter.h

The change in D26502 splits ReaderWriter.h, which contains the APIs
into both the BitReader and BitWriter libraries, into BitcodeReader.h
and BitcodeWriter.h.

Change clang uses to the appropriate split header(s).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@286567 91177308-0d34-0410-b5e6-96231b3b80d8