[llvm] [utils] Add optional --directory flag to update_test_body.py (PR #129562)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Apologies for the late reply. I get what you meant now.

In the output of `clang -c -g a.c`, `DW_AT_comp_dir` encodes the absolute path of PWD.

The utility uses the `PWD=/proc/self/cwd clang -c -g a.c` trick to replace `DW_AT_comp_dir` with /proc/self/cwd on Linux, but it doesn't help macOS, which does not have `/proc/self/cwd`.

```
% grep self/cwd llvm/utils/update_test_body.py
                    PWD="/proc/self/cwd",
```

A portable way is to use `clang -c -g -fdebug-compilation-dir=.`

https://github.com/llvm/llvm-project/pull/129562