[libc++] Rename __bit_reference template parameter to avoid conflict (#80661)

As of 4d20cfcf, `__bit_reference`
contains a template `__fill_n` with a bool `_FillValue` parameter.

Unfortunately there is a relatively widely used piece of scientific
software called NetCDF, which exposes a (C) macro `_FillValue` in its
public headers.

When building the NetCDF C++ bindings, this quickly leads to compilation
errors when the macro interferes with the template in `__bit_reference`.

Rename the parameter to `_FillVal` to avoid the conflict.