Missing information about functions from virtual tables

When decompiling the [attached binary](https://github.com/avast-tl/retdec/files/1705849/classes.zip) (the archive includes the original C++ source code), the generated C file does not contain information about functions from virtual tables.

## Input

```
$ retdec-decompiler.sh classes.exe
```

## Output

The generated C source code (`classes.c`) contains the following code:
```c
struct vtable_4096b8_type g2 = {
.e0 = function_407548
};
struct vtable_4096c8_type g3 = {
.e0 = function_407580
};
struct vtable_4096d8_type g4 = {
.e0 = function_4075d0
};
struct vtable_4096e8_type g5 = {
.e0 = function_407620
};
```
However, there is no information about the referenced functions (e.g. `function_407548`). When I looked into the generated LLVM IR (`classes.c.backend.ll`), there are only declarations of these functions, without bodies.

## Expected output

Since the referenced functions are not from the standard library, their body should be included into the generated C source code. The functions correspond to the following virtual member functions from the original C++ source code:
```cpp
class A {
virtual int foo() { return 1; }
};
class B: public A {
virtual int foo() { return 2; }
};
class C: public A {
virtual int foo() { return 3; }
};
class D: public B, public C {
virtual int foo() { return 4; }
};
```

## Configuration

* Commit: 1db0cb6379bd59a4c885b04eac8bf51a6d0f400f (current `master`)
* 64b Arch Linux, GCC 7.2.1, Debug build of RetDec

Reproduce the issue with classes.exe from the attached classes.zip using retdec-decompiler.sh, then compare classes.c with classes.c.backend.ll. Trace why the virtual-table functions have declarations but no bodies, and verify completion by confirming the generated C includes the bodies for the referenced non-library functions.

Written by the indexing model from the issue text.