Add diagnostic for nested argument constraint

[thomaslevesque]

Related to FakeItEasy/FakeItEasy#1177

Now that nested argument constraints are explicitly forbidden, the analyzer could detect them and issue an error diagnostic.

For instance, this would trigger a diagnostic:

A.CallTo(() => fake.Bar(new Foo(A<int>._))).Returns(42);

Id: FakeItEasy0007
Title: Nested argument constraints are not supported.
Message: The argument constraint A<int>._ is nested in argument new Foo(A<int>._), which isn't supported.
Severity: Error