Fix wchar_t to char conversion warnings

[phprus]

Warnings is introduced in PR #3750

Sample:

C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\xutility(135,50): warning C4244: 'initializing': conversion from 'const _Elem' to '_Ty', possible loss of data [D:\a\fmt\build\test\printf-test.vcxproj]
          with
          [
              _Elem=wchar_t
          ]
          and
          [
              _Ty=char
          ]
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\xutility(134): message : see reference to function template instantiation '_Ty *std::construct_at<_Ty,const _Elem&,void>(_Ty *const ,const _Elem &) noexcept(<expr>)' being compiled [D:\a\fmt\build\test\printf-test.vcxproj]
          with
          [
              _Ty=char,
              _Elem=wchar_t
  enforce-checks-test.cc
          ]

[vitaut]

thank you