[llvm] [DebugInfo] Add DW_OP_LLVM_extract_bits (PR #93990)

[John Brawn via llvm-commits]

john-brawn-arm wrote:

> Can you explain why the signedness matters?

If we have
```
typedef struct __attribute__((packed)) {
  unsigned int x : 4;
  signed int y : 4;
} struct_t;

struct_t g = {1, -1};

int main() {
  auto [x, y] = g;
  return x+y;
}
```
then if y isn't sign-extended, looking at the values in a debugger we have
```
    frame #0: 0x00007ffff7fff164 a.out`main at tmp.cpp:10:10
   7
   8    int main() {
   9      auto [x, y] = g;
-> 10     return x+y;
   11   }
(lldb) p x
(unsigned int) 1
(lldb) p y
(int) 15
(lldb) p x+y
(unsigned int) 16
```
which is wrong. If it's sign-extended we have
```
    frame #0: 0x00007ffff7fff164 a.out`main at tmp.cpp:10:10
   7
   8    int main() {
   9      auto [x, y] = g;
-> 10     return x+y;
   11   }
(lldb) p x
(unsigned int) 1
(lldb) p y
(int) -1
(lldb) p x+y
(unsigned int) 0
```
which is correct.

https://github.com/llvm/llvm-project/pull/93990