[CUDA][HIP] Fix init var diag in temmplate (#69081)

Currently clang diagnoses the following code:
(https://godbolt.org/z/s8zK3E5P5) but nvcc
does not.

`
struct A {
   constexpr A(){}
};

struct  B {
  A a;
  int b;
};

template<typename T>
__global__ void kernel( )
{
   __shared__ B x;
}
`

Clang generates an implicit trivial ctor for struct B, which should be
allowed for initializing a shared variable.

However, the body of the ctor is defined only if the template kernel is
instantiated. Clang checks the initialization of variable in
non-instantiated templates, where it cannot find the body of the ctor,
therefore diagnoses it.

This patch skips the check for non-instantiated templates.