Force visibility of llvm::Any to external

llvm::Any::TypeId::Id relies on the uniqueness of the address of a static
variable defined in a template function. hidden visibility implies vague linkage
for that variable, which does not guarantee the uniqueness of the address across
a binary and a shared library. This totally breaks the implementation of
llvm::Any.

Ideally, setting visibility to llvm::Any::TypeId::Id should be enough,
unfortunately this doesn't work as expected and we lack time (before 12.0.1
release) to understand why setting the visibility to llvm::Any does work.

See https://gcc.gnu.org/wiki/Visibility and
https://gcc.gnu.org/onlinedocs/gcc/Vague-Linkage.html
for more information on that topic.

Differential Revision: https://reviews.llvm.org/D101972

llvm/include/llvm/ADT/Any.h

@@ -23,7 +23,12 @@
 
 namespace llvm {
 
-class Any {
+class LLVM_EXTERNAL_VISIBILITY Any {
+
+  // The `Typeid<T>::Id` static data member below is a globally unique
+  // identifier for the type `T`. It is explicitly marked with default
+  // visibility so that when `-fvisibility=hidden` is used, the loader still
+  // merges duplicate definitions across DSO boundaries.
   template <typename T> struct TypeId { static const char Id; };
 
   struct StorageBase {