[clang][Driver] Implement AddClangSystemIncludeArgs and HasNativeLLVMSupport...

[clang][Driver] Implement AddClangSystemIncludeArgs and HasNativeLLVMSupport for the OpenBSD clang driver.

If not overridden, AddClangSystemIncludeArgs's implementation is empty, so by
default, no system include args are added to the Clang driver. This means that
invoking Clang without the frontend must include a manual -I/usr/include flag,
which is inconsistent behavior. Therefore, override and implement this method
to match. Some boilerplate is also borrowed for handling of the other driver
flags.

While we are here, also override and enable HasNativeLLVMSupport.

Patch by: 3405691582 (dana koch)

Differential Revision: https://reviews.llvm.org/D86412

(cherry picked from commit 2b37174b)