[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

(cherry picked from commit 868ee3f2855615016ba87c14b9444c18a2e1ba30)