<system_error>: Use uglified [[clang::require_constant_initialization]] attribute

[JMazurkiewicz]

Currently <system_error> header uses non-reserved identifiers clang and require_constant_initialization which is incorrect (see #2645):

PS D:\stl-playground> type .\clang_rci.cpp
// Non-reserved identifiers
#define clang
#define require_constant_initialization

#include <system_error> // Whoops!
PS D:\stl-playground> clang .\clang_rci.cpp
In file included from .\clang_rci.cpp:5:
D:\stl\out\build\x64\out\inc\system_error:646:7: error: expected ']'
    [[clang::require_constant_initialization]] static _Ty _Static;
      ^
1 error generated.

This PR simply uglifies this attribute.

[StephanTLavavej]

Neat, I wasn't aware that this was officially documented:

The attribute scope tokens clang and _Clang are interchangeable, as are the attribute scope tokens gnu and __gnu__. Attribute tokens in either of these namespaces can be specified with a preceding and following __ (double underscore) to avoid interference from a macro with the same name. For instance, gnu::__const__ can be used instead of gnu::const.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for noticing and fixing this minor conformance issue! 🐛 🛠️ ✨