The dllimport.cpp test was gating some checks on #ifndef MSABI,
but MSABI was never defined in the test. It seems we are erroring
on code that we should be accepting when compiling for MSVC compatibility.

This should make the test less confusing until PR21406 is fixed.
[clang-tidy] Added -system-headers option.

Added -system-headers option to allow display of warnings from system headers.
This is needed for testing libcxx, for example.