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

[Greg Clayton via llvm-commits]

clayborg wrote:

How would a user currently find the `DW_TAG_LLVM_func_args_changed` DIE for a given `DW_TAG_subprogram` DIE? If a function's gets optimized the DWARF will reflect what is in the program. One possible alternate solution is to have an attribute on the `DW_TAG_subprogram` DIE that points to a function type DIE from the original source. Say we have a function like:

```
int foo(int a, int b, int c);
```
And the return type gets changed to `void` and `int a` gets removed, the DWARF could have the `DW_TAG_subprogram` have a new attribute like `DW_AT_LLVM_original_prototype` that points to what the original source function prototype was. The DWARF would look something like below. The first `DW_TAG_subprogram` DIE @ 0x0000002d is the original function prototype, and the optimized `DW_TAG_subprogram` DIE @ 0x0000005f is the optimized function.
```
0x0000002d:   DW_TAG_subprogram
                DW_AT_type	(0x0000000000000091 "int")

0x0000003d:     DW_TAG_formal_parameter
                  DW_AT_name	("a")
                  DW_AT_type	(0x0000000000000091 "int")

0x00000048:     DW_TAG_formal_parameter
                  DW_AT_name	("b")
                  DW_AT_type	(0x0000000000000095 "float")

0x00000053:     DW_TAG_formal_parameter
                  DW_AT_name	("c")
                  DW_AT_type	(0x0000000000000025 "char")

0x0000005e:     NULL

0x0000005f:   DW_TAG_subprogram
                DW_AT_low_pc	(0x0000000100003ed8)
                DW_AT_high_pc	(0x0000000100003f18)
                DW_AT_frame_base	(...)
                DW_AT_name	("foo")
                DW_AT_LLVM_original_prototype (0x0000002d)

0x0000006b:     DW_TAG_formal_parameter
                  DW_AT_location	(DW_OP_fbreg +12)
                  DW_AT_name	("b")
                  DW_AT_type	(0x0000000000000095 "float")

0x00000076:     DW_TAG_formal_parameter
                  DW_AT_location	(DW_OP_fbreg +8)
                  DW_AT_name	("c")
                  DW_AT_type	(0x0000000000000025 "char")

0x00000081:     NULL
```
If we have the two complete definitions, it would allow BPF trace to do all needed work without any flags as it could just compare the two types and see what changed, like that the function return type has changed and there is no longer an "a" argument.

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