[libcxx] [test] Don't pass INCLUDE to clang via -isystem

Passing the MSVC include dirs via -isystem makes them included before
clang's own include resource dir (<prefix>/lib/clang/<version>/include).
This causes includes of stddef.h to bypass clang's stddef.h which
defines max_align_t, which libc++ needs defined.

This was added in 4372f06d when the
initial windows testing support was added, and has been brought along
since. It's unclear if this was needed back then - now it no longer is
needed at least, and since libc++ started depending on max_align_t, this
became an issue.

Differential Revision: https://reviews.llvm.org/D97167