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