libclang: CXUnsavedFile::length should use a fixed-width type

`CXUnsavedFile` in `clang-c/Index.h` declares `unsigned long Length`.

```C
struct CXUnsavedFile {
const char *Filename;
const char *Contents;
unsigned long Length;
};
```

This varies across ABIs (eg. on big endian architectures), which can cause crashes or incorrect lengths.

Here is an example of a [downstream](https://bugs.openjdk.org/browse/CODETOOLS-7904079) Bug.

The fix will likely require updating the struct to use `uint64_t Length` and adjust internal uses.

The change would intentionally alter libclang's C API layout.