[Patch, Fortran] Implement basic ABSTRACT types

[Daniel Kraft]

Hi,

this patch is a basic implementation of ABSTRACT types (but no DEFERRED 
procedure bindings).  It parses and remembers the ABSTRACT attribute for 
derived-type symbols and checks that no symbol/component is of TYPE(t) 
with t being ABSTRACT, no structure constructor for an ABSTRACT type is 
invoked and no IMPLICIT statement uses an ABSTRACT type.

For the IMPLICIT check, I added logic to remember the loci of all 
IMPLICIT statements to allow a nice diagnostic message to be printed.

In gfc_match_structure_constructor, the tail->val = NULL initialization 
fixes a double-free problem I stumbled across, if anyone wonders :)

I thought about using gfc_use_derived for the ABSTRACT check, but this 
seemed not to work in all cases as I would have liked it to and 
additionally I like the check during resolution much better; and it 
gives IMHO better diagostics for ABSTRACT types in IMPLICIT statements. 
  I hope this is ok so.

What do you think about this patch?  I'm at the moment regression 
testing on GNU/Linux-x86-32 and hopefully no regressions have crept in...

Thanks,
Daniel
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