On Windows, make the release script work with the local git checkout (#78273)

Add two new flags to the release script:
  `--skip-checkout` builds from the local source folder, instead of the downloaded source package
  `--local-python` uses whichever local Python version is installed, instead of a specific version (3.10)

If the LLVM source is already in `C:\git\llvm-project` then one can do in a admin prompt:
```
C:\git>llvm-project\llvm\utils\release\build_llvm_release.bat --version 18.0.0 --x64 --skip-checkout
...
```
This allows for iterating more easily on build issues, just before a new LLVM package is made.

Also fix some warnings on the second stage build (`-Wbackend-plugin`).

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