meson: Allow mismatching RTTI for MSVC

meson: Allow mismatching RTTI for MSVC

This might be safe to relax to all Windows compilers, but I didn't
test Clang or MinGW, so scoping to MSVC for now. For MSVC, this is
safe to mismatch, because the vftables are emitted into all objects
with "pick largest," and the definition with RTTI is larger than the
one without. This is different than the Itanium ABI, which only emits
one copy of the typeinfo in the object which defines the key method.

Acked-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13064>