Understand the inference of calls with decorators

Currently `astroid` doesn't really understand decorators:
```
call = astroid.extract_node("""
def dec(func):
def _():
return func() - 1
return _

@dec
def a():
return 5
a() #@

""")
call.inferred()[0].value
```
The above will return `5` when it should return 4 because of the decorator.

Start by reproducing the example with astroid.extract_node and call.inferred(). The relevant behavior is inference for the decorated function a and its call; compare the current value of 5 with the expected value of 4. Done means decorated calls account for dec and produce the expected inferred result.

Written by the indexing model from the issue text.