__FILE_FULL_PATH__'s behaviour after #line

[Longinus]

Provided not located in the root, those two code samples have 
different behaviours:

```d
#line 42 "file.c"
pragma(msg, __FILE_FULL_PATH__); //outputs the compiler's CWD ~ 
__FILE__
```

```d
#line 42 "/file.c"
pragma(msg, __FILE_FULL_PATH__); //output omits DMD's CWD
static assert(__FILE_FULL_PATH__ == __FILE__); //succeeds
```
Looks like [this 
function](https://github.com/dlang/dmd/blob/master/compiler/src/dmd/root/filename.d#L161) is responsible for this.
Is this the intended behaviour? Is it documented somewhere?
The first behaviour is weird and used to just respond with what's 
written in the `#line` directive, before being [changed by this 
commit](https://github.com/dlang/dmd/commit/1927521a0450c9904a9ed8bada1a2b5f9a506718).