[lld] [lld] Merge equivalent symbols found during ICF (PR #134342)

[Hans Wennborg via llvm-commits]

zmodem wrote:

This has some UB, but I think it illustrates the core of our problem:

```
$ cat /tmp/x.c
const int x = 1;
const int y = 2;
extern const int __attribute__((alias("y"))) alias;

int f() { return *(&x + 1); }
int g() { return alias; }
int h() { return alias * 10; }

#include <stdio.h>
int main() {
  printf("%d %d %d\n", f(), g(), h());
  return 0;
}
```

Before the ICF change:

```
$ build/bin/clang /tmp/x.c -ffunction-sections -fuse-ld=lld -Wl,--icf=all && ./a.out
2 2 20
```

After:

```
$ build/bin/clang /tmp/x.c -ffunction-sections -fuse-ld=lld -Wl,--icf=all && ./a.out
2 2 10
```

https://github.com/llvm/llvm-project/pull/134342