[libc++] Speed up certain locale functions on Windows

The issue is that __libcpp_locale_guard makes some slow calls to setlocale().
This change avoids using __libcpp_locale_guard in snprintf_l().

Fixes https://bugs.llvm.org/show_bug.cgi?id=41131

Differential Revision: https://reviews.llvm.org/D59525

llvm-svn: 356512