[CUDA][HIP] Do not promote constexpr var with non-constant initializer

constexpr var may be initialized with address of non-const variable.
In this case the initializer is not constant in device compilation.
This has been handled for const vars but not for constexpr vars.

This patch makes handling of const var and constexpr var
consistent.

Reviewed by: Artem Belevich

Differential Revision: https://reviews.llvm.org/D119615

Fixes: https://github.com/llvm/llvm-project/issues/53780