[OpenMP] Always apply target declarations to canonical definitions

This patch changes the handling of OpenMP to add the device attributes
to the canonical definitions when we encounter a non-canonical
definition. Previously, the following code would not work because it
would find the non-canonical definition first which would then not be
used anywhere else.

```
int x;
extern int x;
```

This patch now adds the attribute to both of them. This allows us to
perform the following operation if, for example, there were an
implementation of `stderr` on the device.

```
#include <stdio.h>

// List of libc symbols supported on the device.
extern FILE *stderr;
```

Unfortunately I cannot think of an equivalent solution to HIP / CUDA
device declarations as those are done with simple attributes. Attributes
themselves cannot be used to affect a definition once its canonical
definition has already been seen. Some help on that front would be
appreciated.

Fixes https://github.com/llvm/llvm-project/issues/63355

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D153369