Add deleting destructor for c++ ABI

[jayrm]

Implicitly emit the deleting destructor for extern "c++" mangling for better g++ ABI compatibility

In an extern "c++" block, fbc will allocate an entry in the virtual table (if it has one) for the deleting destructor:
Example:

extern "c++"
type T
    __ as integer
    declare constructor
    declare destructor
end type
end extern

When user defines a complete destructor (typical in fbc), the deleting destructor will also be implicitly generated:

'' pseudo code for illustration only
destructor T alias "T.D0" ()
if( @this <> NULL ) then
delete this
end if
end destructor

Where the delete operator will first invoke the complete destructor (~T::T()) followed by deallocating the memory.

Some simple tests have been added to check the usage of passing an fbc declared type to a g++ implemented class, however the tests are quite simple and do not fully explore virtual tables and inheritance. More testing is needed. However, the change should not adversely effect non extern "c++" source codes.

Note: without support for thiscall calling convention, this is not expected to work on 32-bit windows where the default calling convention is thiscall for c++ classes by default.

References:
sf.net: #923 Incompatibility in ABI destructor with GCC C++
fb.net: C++ ABI Compatible?

Background:
Previously, fbc did not add the deleting destructor to the type's virtual table making it difficult to use (some) g++ compiled code directly.