Fix CI warnings in own float parser

- pow5_table.hpp: pow5_128_largest_power was unused in this branch's
  own code (GCC -Werror=unused-const-variable); tie it to the table
  size with a static_assert instead of removing it, since a later
  branch in the stack (json-view/23-zmij) uses it.
- number_parse.hpp: rename the local variable `copy` to `buffer` to
  satisfy cpplint's build/include_what_you_use check.
- unit-class_lexer.cpp: extend the NOLINT list on the seeded mt19937
  with bugprone-random-generator-seed, and parenthesize
  `8 * sizeof(Bits) - 1` for clang-tidy.

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

include/nlohmann/detail/input/number_parse.hpp

@@ -987,10 +987,10 @@ void convert_float_locale_aware(StringType& token, std::size_t decimal_point_pos
         }
         else
         {
-            std::string copy(token.data(), token.size());
-            copy.replace(decimal_point_position, 1, decimal_point);
-            strtof_by_type(value, copy.c_str(), &endptr);
-            complete = endptr == copy.c_str() + copy.size();
+            std::string buffer(token.data(), token.size());
+            buffer.replace(decimal_point_position, 1, decimal_point);
+            strtof_by_type(value, buffer.c_str(), &endptr);
+            complete = endptr == buffer.c_str() + buffer.size();
         }
 
         if (JSON_HEDLEY_LIKELY(complete))