Private functions which are never used could be removed by the GCC backend, but
we somehow prevent the backend from doing that.

Example: C
--------------
static void foo() {}
--------------
gcc -S -O test.c

D:
--------------
private void foo() {}
--------------
gdc -S -O test.c

The problem is probably in d-decls.cc(FuncDeclaration::toSymbol) and is one of
(or a combination of) these: TREE_PUBLIC set, TREE_USED set, TREE_STATIC not
set?