[Patch, Fortran] PR 46974: Handle C_(FUN)PTR variables in TRANSFER

[Tobias Burnus]

The patch adds compile-time simplification support for C_PTR/C_FUNPTR 
variables. Those are special beasts as they are derived types in Fortran 
(gfc_expr/gfc_symbol) but for the middle end (tree) they are simple 
(function) pointers.

The patch add special treatment to the <transfer bits> -> 
C_(FUN)PTR-type conversion in target-memory.c - and it handles them 
explicitly in assignment. I removed the bits about C_NULL_PTR which 
seems to be dead code. I believe the function is never called for 
C_NULL_PTR as the conversion to an c_intptr_t-like integer happens 
earlier. Additionally, even if for C_NULL_PTR the code block would be 
entered, there were a segfault as the component->backend_decl is NULL. 
-- At least that's how I read gfc_get_derived_type.

Build and regtested on x86-64-linux.
OK for the trunk?

Tobias

PS: I wrote in the PR that one should try to avoid the special handling 
- obviously, I failed. However, the patch at least fixes the PR and 
allows compile-time simplification.
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