[lld] [lld] Merge GOT entries for symbols that have been ICFed (PR #131630)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Please could you write a full description for the commit message? There's a lot of subtle details about why this occurs and why this is the best fix.
> 
> It also helps people reading the commit log to not have to refer to an external URL for details.
> 
> I'm surprised the outliner has chosen to split apart an ADRP and a ldr. This prevents the relocation optimization
> 
> ```
> ADRP  x0, :got: symbol            // R_<CLS>_ADR_GOT_PAGE
> LDR   x0, [x0 :got_lo12: symbol]  // R_<CLS>_LD64_GOT_LO12_NC
> 
> // after optimization:
> 
> ADRP  x0, symbol                  // R_<CLS>_ADR_PREL_PG_HI21
> ADD   x0, x0, :lo12: symbol       // R_<CLS>_ADD_ABS_LO12_NC
> ```
> 
> Defined in [ARM-software/abi-aa@`main`/aaelf64/aaelf64.rst#579relocation-optimization](https://github.com/ARM-software/abi-aa/blob/main/aaelf64/aaelf64.rst#579relocation-optimization)
> 
> It may be worth raising an issue to stop the outliner from doing that.

There is a wall of text in the current description. Instead of spending a lot of words explaining the subtle details, it might be clearer to describe the linker output and why it's problematic. I've left a short description at https://github.com/llvm/llvm-project/issues/129122#issuecomment-2739009435 , which could be useful to make the commit message shorter.

https://github.com/llvm/llvm-project/pull/131630