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

[via llvm-commits]

yonghong-song wrote:

> > Wouldn't it be possible to set the existing DW_AT_artificial attribute instead of introducing vendor-specific extensions?
> 
> not sure if that'd connote either of the properties being expressed here (uncallable, and un-printable-returns) - for instance in C++, implicit ctors, assignment operators, and dtors are artificial but callable/printable returns based on ABI, so we couldn't have debuggers assuming artificial functions can't be called/have their return values printed.

I am not an expert in DW_AT_artificial. In dwarf doc, I see
```
A compiler may wish to generate debugging information entries for objects or types that
were not actually declared in the source of the application. An example is a formal
parameter entry to represent the hidden this parameter that most C++ implementations
pass as the first argument to non-static member functions.
Any debugging information entry representing the declaration of an object or
type artificially generated by a compiler and not explicitly declared by the source
program may have a DW_AT_artificial attribute, which is a flag.
```
and flag
```
A flag is represented explicitly as a single byte of data (DW_FORM_flag) or
implicitly (DW_FORM_flag_present). In the first case, if the flag has value
zero, it indicates the absence of the attribute; if the flag has a non-zero
value, it indicates the presence of the attribute. In the second case, the
attribute is implicitly indicated as present, and no value is encoded in the
debugging information entry itself.
```

Does this enable coding can cover all the below case:
  - return value is discarded.
  - arguments are changed.
  - both return value discarded and arguments changed.
?

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