[lld] [ELF] adjustOutputSections: don't copy SHF_EXECINSTR when an output does not contain input sections (PR #70911)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

I think this is an improvement and the code-changes look good to achieve what is in the specification.

I don't think that this change would fully address the change in https://discourse.llvm.org/t/lld-output-section-flag-assignment-behavior/74359 as that had:
```
[Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
  [ 1] .code             PROGBITS        0000000010000000 001000 000003 00  AX  0   0  1
  [ 2] .CPU0.heap        NOBITS          0000000010000040 001003 000400 00  AX  0   0 64
  [ 3] .CPU1.heap        NOBITS          0000000010000440 001003 000400 00  AX  0   0 64
  [ 4] .CPU2.heap        NOBITS          0000000010000840 001003 000400 00  AX  0   0 64
```
With this change I think we would have flags of A instead of AX. Leaving a non writeable BSS section which looks a bit odd.

If we start from the assumption that OutputSections with just symbol assignments are most likely to be data then dropping SHF_EXECINSTR makes sense. In the general case (SHT_PROGBITS) we can't tell if the user intended them to be read-only or read-write so we can't justify adding SHF_WRITE. However if the user has used NOLOAD or TYPE=SHT_NOBITS then I think there is a case for adding SHF_WRITE.

I don't have a particularly strong opinion here so happy to go with your original patch if you prefer.

https://github.com/llvm/llvm-project/pull/70911