Linux build improvements - clang8, no custom libcxx

[madratman]

• Tested with UE4.24, Ubuntu 18 (might have to enforce 4.24, right now we support >=4.22)
  • or add option for clang-7 for UE 4.22
• install clang-, libc++-8-dev, libc++abi-8-dev in setup.sh
• remove all the clang5.0 and custom libcxx download and building
• in commonsetup.cmake, throwing in a -stdlib=libc++ and removing all the unreal specific flags seems to the job

[rajat2004]

PRs working properly for me, manually removed the llvm-build directory, but no problems
Some new warnings are there, which is expected when upgrading to a new compiler
Travis build of the current PR - https://travis-ci.com/github/rajat2004/AirSim/builds/157408468

[rajat2004]

@madratman I've opened madratman#1 against this branch on your fork, which updates the compiler for OSX
Travis build - https://travis-ci.com/github/rajat2004/AirSim/builds/157476649

[rajat2004]

Also, what should be done with the Unity compilation? That is still using clang-5

[madratman]

travis is working for upstream airsim as well - https://travis-ci.org/github/microsoft/AirSim/builds/669789574 / https://travis-ci.org/github/microsoft/AirSim/pull_requests
but for some reason, not showing up on gh PR page