Fix the MSVC and GCC builds of develop (C4127, -Weffc++) (#6016)

json_view number(): #6006 folded the check of the digit after a minus
sign into `else if (!negative || ...)`, which MSVC 19.44 reports as
C4127 (conditional expression is constant) and /WX turns into an
error in every MSVC and clang-cl job. The check now sits in the
`if (negative)` block, right after the sign is skipped; the error and
its position are unchanged.

unit-locale-cpp.cpp: the locale_switching_token added in #5997 held a
`const char*`, so GCC's -Weffc++ asked for a copy constructor and copy
assignment. The locale name is now a std::string.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json_view.hpp

@@ -2473,20 +2473,20 @@ indent_done:
             if (negative)
             {
                 ++p;
+                if (NLOHMANN_VIEW_UNLIKELY(cur() < '0' || cur() > '9'))
+                {
+                    return fail(error_code::number_after_minus);
+                }
             }
             const unsigned char* const int_start = p;
             if (cur() == '0')
             {
                 ++p;
             }
-            else if (!negative || NLOHMANN_VIEW_LIKELY(cur() >= '1' && cur() <= '9'))
-            {
-                // (without a sign, the dispatch saw a digit)
-                p = skip_digits(p + 1, e);
-            }
             else
             {
-                return fail(error_code::number_after_minus);
+                // (a digit: the dispatch or the check after the sign saw it)
+                p = skip_digits(p + 1, e);
             }
             const auto int_digits = static_cast<std::size_t>(p - int_start);
             std::size_t frac_digits = 0;