[libc++] Rename __bit_reference template parameter to avoid conflict
As of
4d20cfcf4eb08217ed37c4d4c38dc395d7a66d26, `__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.
PR: 276104
MFC after: 1 month