[Clang][Cygwin] Fix symbol visibility definition (#138118)
Currently building for Cygwin hits this error:
```
In file included from /h/projects/llvm-project/clang/lib/Basic/Attributes.cpp:17:
/h/projects/llvm-project/clang/include/clang/Basic/ParsedAttrInfo.h:180:73: error: invalid declarator before ‘;’ token
180 | extern template class CLANG_TEMPLATE_ABI Registry<clang::ParsedAttrInfo>;
```
That's because `CLANG_TEMPLATE_ABI` ends up not being defined. The
solution here is to follow MinGW case.
