fix lli bug in llvm-4.0

[yufengzjj]

when build lli, readMem return value type(Expected<std::vector<uint8_t>>) mismatch its sig(Expected<std::vector<char>>),cause a compiler error

D:/msys64/home/DELL/obfuscator/include/llvm/ExecutionEngine/Orc/OrcRemoteTargetClient.h:696:26: error: could not convert '((llvm::orc::remote::OrcRemoteTargetClient*)this)->callBllvm::orc::remote::OrcRemoteTargetRPCAPI::ReadMem(Src, Size)' from 'Expected<vector<unsigned char,allocator>>' to 'Expected<vector<char,allocator>>'
return callB(Src, Size);

it can be checked by gcc-8.2.0(8.0.x and above) but not in old gcc version(used to be a bug for gcc?).

affect llvm4.0-llvm5.0

llvm bug ref:https://bugzilla.redhat.com/show_bug.cgi?id=1540620