C++20 support - continue to use c++17 on files that use LLVM

[spoonincode]

LLVM headers prior to version 13.0 result in an error when compiling in c++20 mode.

Refactor EOS VM OC a wee bit to isolate LLVM headers to just a few .cpp files (and eliminate usage of "other stuff", like fc, from those same files), and compile those with c++17 still.

Compared to #1116, the positive with this change is that it retains compatibility with existing LLVM versions and doesn't get us in the business of patching system provided files which may cause some additional subtle headaches (do they need to be installed for libtester? do they cause an ODR violation?)

The negative is mainly that it's not clear how long lived this solution can be. For example, once we tackle #673, LLVM & EOS VM will be more tightly coupled. EOS VM doesn't use c++20 today, but it might in the future.

I don't particularly like using COMPILE_FLAGS "--std=gnu++17" but there is no source file property for CXX_STANDARD; there is a target property for CXX_STANDARD though, so a wee bit of clean up could possibly use that instead.

[greg7mdp]

@spoonincode we need to merge either this PR or #1116 in order to switch to C++20. Let's decide and move forward.