[c2cpg] Proper C++ constructor calls

[max-leuthaeuser]

This PR adds support for C++ constructors and constructor calls.
See: https://en.cppreference.com/w/cpp/language/constructor

Both constructors and C++17 style member initializer lists are supported.

This includes but is not limited to the following improvements:

• detect methods that are constructors (was missing before)
• provide the implicit this parameter
• create member initialization in those methods incl. synthetic this access if needed
• create proper calls (i.e., call methodFullNames) to constructors for new expressions as well es initializer list syntax expressions
• also create calls to default constructors if needed. E.g.,:

S s;      // calls S::S()
S s2(10); // calls S::S(int)

• handle externally defined constructors by putting them under the proper TypeDecl in a later pass. E.g.,:

class Foo {
  public:
    Foo(int i) {};
};

Foo::Foo(int i, int j) {}; // will be put under Foo 
Bar::Bar(float x) {}; // Bar is not defined at all; keep it here so no dangling methods