Cleanup includes for LLVMBitstream

This is a very small library, so only a few changes and very little gain.

$ clang++ -E  -Iinclude -I../llvm/include ../llvm/lib/Bitstream/Reader/*.cpp -std=c++14 -fno-rtti -fno-exceptions | wc -l
before: 65777
after:  64997

Related Discourse thread: https://llvm.discourse.group/t/include-what-you-use-include-cleanup

Differential Revision: https://reviews.llvm.org/D118373