Summary:
FunctionDefinition::definition always looks for FunctionDefinition based on the DeclarationId, but we don't need to do that if we already have a FunctionDefinition.
This could cause problems if we have multiple (same) definitions.

Test Plan:
- create a small cmake (or whatever) project with 3 targets
-- util1.c / util2.c / util3.c (same content):

```
#include <stdio.h>
#include <stdlib.h>

void configure()
{
    printf("do stuff in %s\n", __FILE__);
}

int main(int argc, char *argv[])
{
    configure();
    puts("Hello, UTIL!");
    exit(EXIT_SUCCESS);
}
```

-- CMakeLists.txt:
```
add_executable(util1 util1.c)
add_executable(util2 util2.c)
add_executable(util3 util3.c)
```

- open util1.c
- open the outline and select "configure"
- EXPECTED: it should open the configure in util1.c (or the currently opened file)
- repeat with util2 and util3
- ACTUAL: only one of them works, the other two point to the wrong file.

Reviewers: #kdevelop, mwolff

Reviewed By: #kdevelop, mwolff

Subscribers: mwolff, kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D16356