[clang] [llvm] Add DW_ATE_GNU_complex_signed to fix assertion (PR #161695)

[David Blaikie via llvm-commits]

dwblaikie wrote:

Not sure how useful it is, but apparently binutils dwarfdump dumps this as:
```
< 1><0x00000059>    DW_TAG_base_type <abbrev 4>
                      DW_AT_byte_size             8
                      DW_AT_encoding              DW_ATE_ALTIUM_fract
                      DW_AT_name                  complex int
```
(using gcc to compile an example with `_Complex  int`)
& I see some mention of/discussion of this here (20 years ago): https://dwarfstd.org/pipermail/dwarf-discuss/2006-March/000260.html - not sure if it's intentional that this encoding is that one, or whether there's a numbering collision. Could check with a GCC person, or dwarf-discuss, or perhaps Dave Anderson himself.. 

Be nice to know how GCC emits this, whether it uses the ATE_ATIUM_fract name, or some other name for the same constant.

https://github.com/llvm/llvm-project/pull/161695