[mlir] Fix TypeID for shared libraries built with -fvisibility=hidden.

* Isolates the visibility controlled parts of its implementation to a detail namespace.
* Applies a struct level visibility attribute which applies to the static local within the get() functions.
* The prior version was not emitting a symbol for the static local "instance" fields when the user TU was compiled with -fvisibility=hidden.

Differential Revision: https://reviews.llvm.org/D89153