[libc++][modules] Fixes clang-tidy exports. (#76288)

As suggested in #71438 we should use
  export import std;
in the std.compat module.

Using this exports some named declarations from functions and records,
adding them to the global namespace. Clang correctly does not export
these and it's an issue in the declaration filtering. Declarations in
function or record context are not considered a global named
declaration.