type name first seen using 'struct' now seen using 'class'

[jlfaucher]

Using ICU4X, the Microsoft compiler reports these warnings when using the option warnings level 2 /W2:

(compiling the ICU4X C++ FFI example for locale)

cl /Zi /std:c++20 /EHsc /W3 test.cpp ..\..\..\..\..\target\debug\icu_capi_staticlib.lib userenv.lib advapi32.lib ws2_32.lib bcrypt.lib
Microsoft (R) C/C++ Optimizing Compiler Version 19.33.31630 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

test.cpp
E:\Local\Unicode\ICU4X\git\ffi\diplomat\cpp\examples\locale\../../include/ICU4XLocale.hpp(14): warning C4099: 'ICU4XLocale': type name first seen using 'struct' now seen using 'class'
E:\Local\Unicode\ICU4X\git\ffi\diplomat\cpp\include\diplomat_result_box_ICU4XLocale_ICU4XError.h(9): note: see declaration of 'ICU4XLocale'
E:\Local\Unicode\ICU4X\git\ffi\diplomat\cpp\examples\locale\../../include/ICU4XLocale.hpp(32): warning C4099: 'ICU4XLocale': type name first seen using 'struct' now seen using 'class'
E:\Local\Unicode\ICU4X\git\ffi\diplomat\cpp\examples\locale\../../include/ICU4XLocale.hpp(32): note: see declaration of 'ICU4XLocale'

The warning could be avoided if the struct is declared into the namespace capi.

Example of generated code:
diplomat_result_box_ICU4XLocale_ICU4XError.h

typedef struct ICU4XLocale ICU4XLocale;
#include "ICU4XError.h"
#ifdef __cplusplus
namespace capi {
extern "C" {
#endif
    typedef struct diplomat_result_box_ICU4XLocale_ICU4XError {
    union {
        ICU4XLocale* ok;
        ICU4XError err;
    };
    bool is_ok;
} diplomat_result_box_ICU4XLocale_ICU4XError;
#ifdef __cplusplus
} // extern "C"
} // namespace capi
#endif

[Manishearth]

Yeah, our C++ headers need a bunch of improvement, planned as a part of #280