Cleanup LLVMRemarks includes

Based on the output of include-what you-use.

Most notably, llvm/Remarks/Remark.h is no longer automatically included by
llvm/Remarks/RemarkParser.h, so client code may need to include explicitly.

clang++ -E  -Iinclude -I../llvm/include ../llvm/lib/Remarks/*.cpp -std=c++14 -fno-rtti -fno-exceptions | wc -l
before: 770253
after:  759347

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

Differential Revision: https://reviews.llvm.org/D118506