Fix std::byte namespace declaration

[François Dumont]

Current build of libstdc++ with --enable-symvers=gnu-versioned-namespace 
fails (at least under Linux) because of:

In file included from 
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/memory_resource:39,
 Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  from 
../../../../../git/libstdc++-v3/src/c++17/memory_resource.cc:25:
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/cstddef:71:59: 
error: la référence à « byte » est ambiguë
 Â Â  71 |Â Â  template<> struct __byte_operand<bool> { using __type = byte; };
 Â Â Â Â Â  | ^~~~
In file included from 
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/stl_algobase.h:61,
 Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  from 
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/memory:62,
 Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  from 
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/memory_resource:37,
 Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  from 
../../../../../git/libstdc++-v3/src/c++17/memory_resource.cc:25:
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/cpp_type_traits.h:395:30: 
note: les candidats sont : « enum class std::__8::byte »
 Â  395 |Â Â  enum class byte : unsigned char;
 Â Â Â Â Â  |Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  ^~~~
In file included from 
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/memory_resource:39,
 Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  from 
../../../../../git/libstdc++-v3/src/c++17/memory_resource.cc:25:
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/cstddef:68:14: 
note:                      « enum class std::byte »
 Â Â  68 |Â Â  enum class byte : unsigned char {};
 Â Â Â Â Â  |Â Â Â Â Â Â Â Â Â Â Â Â Â  ^~~~

I think the issue if that std::byte declaration in cpp_type_traits.h has 
been done in versioned namespace, so the attached patch.

 Â Â Â  * include/bits/cpp_type_traits.h (std::byte): Move outside versioned
 Â Â Â  namespace.

Tested under Linux x86_64.

Ok to commit ?

François

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