[mlir] Mark ClassID::getID as LLVM_EXTERNAL_VISIBILITY to ensure only one version exists

[rriddle]

Summary

ClassID is used as a type id and must be unique in the face of shared libraries to ensure correctness. This fixes failures related to BUILD_SHARED_LIBs on macos.

[jroelofs]

LGTM

mlir/include/mlir/Support/STLExtras.h

inline void interleaveComma(const Container &c, raw_ostream &os) {
   interleaveComma(c, os, [&](const T &a) { os << a; });
 }
 
 /// A special type used to provide an address for a given class that can act as
 /// a unique identifier during pass registration.
 /// Note: We specify an explicit alignment here to allow use with PointerIntPair
 /// and other utilities/data structures that require a known pointer alignment.
 struct alignas(8) ClassID {
-  template <typename T> static ClassID *getID() {
+  template <typename T>
+  LLVM_EXTERNAL_VISIBILITY static ClassID *getID() {
     static ClassID id;
     return &id;
   }
-  template <template <typename T> class Trait> static ClassID *getID() {
+  template <template <typename T> class Trait>
+  LLVM_EXTERNAL_VISIBILITY static ClassID *getID() {
     static ClassID id;
     return &id;
   }
 };
 
 /// Utilities for detecting if a given trait holds for some set of arguments
 /// 'Args'. For example, the given trait could be used to detect if a given type
 /// has a copy assignment operator: