[OpenMP] Make Ctor / Dtor functions have external visibility

The default construction of constructor functions by LLVM tends to make
them have internal linkage. When we call a ctor / dtor function in the
target region we are actually creating a kernel that is called at
registration. Because the ctor is a kernel we need to make sure it's
externally visible so we can actually call it. This prevented AMDGPU
from correctly using constructors while NVPTX could use them simply
because it ignored internal visibility.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D122504