[lldb] lldb evaluates generic DW_OP_abs and DW_OP_shra with the wrong signedness

This is related to a previously reported LLDB `DW_OP_mod` issue: #207015

The common pattern is that LLDB appears to let the incidental signedness stored in its internal scalar value influence DWARF generic-operand evaluation. For some opcodes, this does not change the final bit pattern, but it does change the result for at least `DW_OP_abs` and `DW_OP_shra`.

[DWARF v5](https://dwarfstd.org/doc/DWARF5.pdf) Section 2.5.1.4 gives these operations explicit signed semantics:

- `DW_OP_abs` interprets the top stack entry as a signed value before taking the absolute value.
- `DW_OP_shra` performs an arithmetic right shift, keeping the sign of the shifted value.

GDB evaluates the examples below according to those rules. LLDB does not.

## Minimal Examples

### DW_OP_abs

```text
DW_OP_const8u 0xffffffffffffffff
DW_OP_abs
DW_OP_stack_value
```

On a 64-bit target, the correct `DW_OP_abs` interpretation is signed:

```text
DW_OP_const8u 0xffffffffffffffff -> generic all-ones value
DW_OP_abs -> abs(-1) = 1
```

- Expected result: `1`
- GDB result: `1`
- LLDB result: `-1`

LLDB instead keeps the value as unsigned all-ones:

```text
abs(UINT64_MAX) = UINT64_MAX
```
If that final all-ones value is later materialized and printed as a signed source-level integer, it is displayed as `-1` instead of `1`.

### DW_OP_shra

```text
DW_OP_const8u 0xffffffffffffffff
DW_OP_const1u 63
DW_OP_shra
DW_OP_stack_value
```

On a 64-bit target, the shifted value should be interpreted as signed `-1`:

```text
DW_OP_const8u 0xffffffffffffffff -> generic all-ones value
DW_OP_const1u 63
DW_OP_shra -> -1 >> 63 = -1
```

- Expected result: `-1`
- GDB result: `-1`
- LLDB result: `1`

LLDB instead behaves as if it shifted the value unsigned:

```text
UINT64_MAX >> 63 = 1
```

So LLDB returns `1` instead of `-1`.

Start by reproducing the two minimal DW_OP_abs and DW_OP_shra examples in LLDB and compare their results with the DWARF v5 rules and GDB. Trace LLDB's generic DWARF-operand evaluation and internal scalar signedness; the work is done when both examples produce 1 and -1 respectively.

Written by the indexing model from the issue text.