[clang-scan-deps] Implement P2223R2 for DependencyDirectiveScanner.cpp (#143950)

P2223R2 allows the line-continuation slash `\` to be followed by
additional whitespace. The Clang lexer already follows this behavior,
also for versions prior to C++23. The dependency directive scanner
however only implements it for `#define` directives (15d5f5d).

This fully implements P2223R2 for the dependency directive scanner (for
any C++ standard) and aligns the dependency directive scanner's splicing
behavior with that of the Clang lexer.

For example, the following code was previously not scanned correctly by
`clang-scan-deps` but now works as expected:

```cpp
import \<whitespace here>
A;
```