Replace 'magic static' with a member variable for SCYL kernel names

I discovered when merging the __builtin_sycl_unique_stable_name into my
downstream that it is actually possible for the cc1 invocation to have
more than 1 Sema instance, if you pass it multiple input files, each
gets its own Sema instance and thus ASTContext instance.  The result was
that the call to Filter the SYCL kernels was using an
ItaniumMangleContext stored via a 'magic static', so it had an invalid
reference to ASTContext when processing the 2nd failure.

The failure is unfortunately flakey/transient, but the test that fails
was added anyway.

The magic-static was switched to a unique_ptr member variable in
ASTContext that is initialized when needed.