[Patch,Fortran] PR39427/37829 - implement F2003's constructors

[Tobias Burnus]

Dear all,

this patch fixes as collateral effect PR 37829 (alias PR 45190) where 
C_PTR/C_FUNPTR occurred when use associating a module using them, if one 
additionally uses iso_fc_binding directly.

The main part of this patch, however, is for PR 39427 (alias 45190): 
Allowing generic functions to have the same name as a derived type, 
which is a Fortran 2003 feature. In expressions, the generic functions 
have a higher precedence then the structure constructor. Note that the 
functions are not required to return the derived type.

This feature allows one to create something which looks similar to 
constructors in other OOP languages, except that static constructor 
functions do not exist.

This patch implements them by creating for each derived type two symbols 
(symtrees): One for the derived type and one for the generic function, 
which links to the derived type. To distinguish them, the derived type 
starts with a capital letter in the symtree. In order to facilitate the 
error-message handling, the symbol itself remains in lower case.

The main challenges were to ensure that one gets the derived type when 
needed and to store them properly in the module. The most time consuming 
part was to find all the places one had to change that issues with 
module reading could turn up much later; for instance at resolution time 
of a scope which had read that module. In total, it took 18 months 
between the first draft patch (cf. PR39427 comment 6, 12-14) and the 
final patch. Although, the patch looked almost working by then, it took 
many, many, many hours to fix the issues. Also the RFC patch, posted 6 
days ago, had more issues than I had hoped for.

The attached patch had been build on x86-64-linux and successfully 
regtested (gfortran and libgomp). (A full bootstrap of an almost-ready 
version was done as well; I had to rebuild because I found some 
left-over commented code blocks.)

Additionally, I tried the previous patches with several programs to 
reduce the likelihood that it breaks real-world code. In particular, the 
very latest version of the patch was used to compile FLEUR, Elk, Octopus 
and the Polyhedron benchmark. Yesterday evening's version was used to 
compile the Exciting code (which includes the sensitive FoX Fortran XML 
library), CP2K, PSBLAS and FGSL. With a slightly older version, I also 
successfully compiled Tonto, Quantum Espresso and Abinit.

OK for the trunk?

Tobias

PS: I have also included a patch for the website, i.e. 
http://gcc.gnu.org/gcc-4.7/changes.html#fortran

PPS: As mentioned in the attachment, the patch includes the tree-walking 
patch, which was posted before. It's a really an independent bug, even 
if it only exposed with the constructor patch. I can either commit it 
before or as part of this patch. See also 
http://gcc.gnu.org/ml/fortran/2011-11/msg00026.html
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