Cleanup llvm/utils/TableGen headers

Based on the output of include-what-you-use.
It's an utility directory, so no much impact on other code areas.

clang++ -E  -Iinclude -I../llvm/include ../llvm/utils/TableGen/*.cpp -std=c++14 -fno-rtti -fno-exceptions | wc -l
before: 4327274
after:  4316190

Related discourse thread: https://llvm.discourse.group/t/include-what-you-use-include-cleanup
Differential Revision: https://reviews.llvm.org/D118466