[clang] [llvm] [ThinLTO][AIX] Add ThinLTO support for #pragma comment(copyright, ...) (PR #199398)

[Wael Yehia via cfe-commits]

w2yehia wrote:

Tony and I spoke on Friday, the fix looks good, one point to get a second opinion is the linkage of the comment string variable. 
To add to the [previous](https://github.com/llvm/llvm-project/pull/199398#issuecomment-4578593503) comment, in practice AIX seems to include filenames in the copyright string, so the de-duplication will not benefit them.

Also, if `internal` works for the thinLTO case (i.e. it doesn't prohibit functions from getting imported), it's always better to internalize than not. Tony was going to check if `internal` works. I think it should given that this similar test allows `foo` to be imported:
```
// foo.c
static int x;
int *foo() { return &x; }  // foo references the internal `x`
// main.c
int *foo();
int main() { return *foo(); }
```

https://github.com/llvm/llvm-project/pull/199398