move attribute after enum class to fix clang warning (#20198)

Building with clang produce the following error:
text_format.h:55:1: warning: attribute 'visibility' is ignored, place it after "enum class" to apply attribute to type declaration [-Wignored-attributes]

The warning occurs because attributes like
`__attribute__((visibility("default")))` are ignored when placed before an `enum class`. This is due to how attributes are parsed in C++.

Moving the attribute after `enum class` ensures it is applied correctly to the type declaration.

Closes #20198

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/20198 from clementperon:main a48ecebea76aed5a4b0fbc3f7f54923b3b4bb02c
PiperOrigin-RevId: 724338159