[libcxx] [test] Fix the include_as_c.sh.cpp test in MSVC configurations

Avoid including a header that is known not to work with clang in MSVC
mode when compiling as C.

(Alternatively, this could be something like "XFAIL: clang && msvc",
but I think it's more useful to actually check the rest of the test
instead of expecting the whole test to fail.)

Differential Revision: https://reviews.llvm.org/D103400