Cleanup LLVMTextAPI headers

Based on the output of iwyu. A full rebuild of llvm-project doesn't exhibit any
significant false dependencies.

The impact on preprocessed output is larger than expected, given the small
amount of changes

$ clang++ -E  -Iinclude -I../llvm/include ../llvm/lib/TextAPI/*.cpp -std=c++14 -fno-rtti -fno-exceptions | wc -l
before: 635319
After: 643716

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