Multiline line directives produce incorrect line notes

```c++
#line 6 "path\
/to\
/file"
int x;
```

`int x` is expected to be on line 6 but `clang -E` produces:

```c++
...
# 8 "path/to/file"
int x;
```
While gcc:
```c++
...
# 6 "path/to/file"
int x;
```
https://godbolt.org/z/6xEY7Y7en

The same for gnu digit directives:

```c++
#9 \
"file" \
3
int gnu_x;
```

Start by running the multiline #line and GNU digit-directive examples with clang -E, using the linked Compiler Explorer reproduction and comparing the output with GCC. Trace the Clang preprocessor handling of these directives; done means int x and int gnu_x receive the expected source line and filename notes.

Written by the indexing model from the issue text.