[llvm] [Dwarf][Transforms] Add dwarf support when func signature changed (PR #127855)

[via llvm-commits]

yonghong-song wrote:

I have discussed with @jemarch about one possible dwarf format which does not need new attribute/tag and yet can represent modified func signatures.

```
DW_TAG_subprogram
                DW_AT_low_pc	(0x0000000100003ed8)
                DW_AT_high_pc	(0x0000000100003f18)
                DW_AT_frame_base	(...)
                DW_AT_name	("foo")

                DW_AT_artificial (refers to "DW_TAG_inlined_subroutine", see below)

                DW_TAG_formal_parameter
                   DW_AT_name	("a")
                   DW_AT_type	(0x0000000000000091 "int")

                DW_TAG_formal_parameter
                   DW_AT_location
                   DW_AT_name	("b")
                   DW_AT_type	(0x0000000000000095 "float")

                DW_TAG_formal_parameter
                   DW_AT_location
                   DW_AT_name	("c")
                   DW_AT_type	(0x0000000000000025 "char")

NULL
```
And
```
       DW_TAG_inlined_subroutine     
                DW_AT_type	(0x0000000000000091 "int")

                DW_TAG_formal_parameter
                  DW_AT_name	("b")
                  DW_AT_type	(0x0000000000000091 "int")

               DW_TAG_formal_parameter
                  DW_AT_name	("c")
                  DW_AT_type	(0x0000000000000095 "float")
```

DW_AT_artificial suggests the new signature is generated by compiler optimizations and it is generated only when func signature gets changed.

In this case, we need to generate dwarf for changed func signature after middle end optimization and before SelectionDAG. I see middle-end does have simple dwarf builder but I didn't find complex builder to reconstruct dwarf types based on IR types. Any suggestion on how to generate dwarf types based on IR types in middle-end?

https://github.com/llvm/llvm-project/pull/127855