[llvm] Remove (some) LLVMDemangle header dependencies

- Avoid using <iterator> for std::end on a plain array (using <array> instead)
- Avoid using <algorithm> for std::min and std::equal (using alternate logic and std::strcmp instead)

As an hint to the impact of the cleanup, running

clang++ -E  -Iinclude -I../llvm/include ../llvm/lib/Demangle/*.cpp -std=c++14 -fno-rtti -fno-exceptions | wc -l

before:  203965 lines
after:   169704 lines