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