(imcomplete) support concurrent_assertion_statement in entity_statement_part

id like to support this useful assertion.
could you mind to guide me to win?
```
$ make check
[SNIP]
Running suite(s): parse
100%: Checks: 26, Failures: 0, Errors: 0
PASS: test_parse.exe
[SNIP]
        assert3 : failed
assertion "item->tree != NULL" failed: file "tree.c", line 1009, function: tree_ref
[SNIP]

$ export NVC_LIBPATH=./lib/std:./lib/ieee
$ gdb ./src/nvc.exe
GNU gdb (GDB) 7.6.50.20130728-cvs (cygwin-special)
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..
Reading symbols from /home/hiyuh/git-repos/nvc/src/nvc.exe...done.
(gdb) b tree_ref
Breakpoint 1 at 0x45cf07: file tree.c, line 1008.
(gdb) r -a ./test/regress/assert3.vhd
Starting program: /home/hiyuh/git-repos/nvc/src/nvc.exe -a ./test/regress/assert3.vhd
[New Thread 1756.0x660]
[New Thread 1756.0xdc4]

Breakpoint 1, tree_ref (t=0x80051998) at tree.c:1008
1008       item_t *item = lookup_item(t, I_REF);
(gdb) bt
#0  tree_ref (t=0x80051998) at tree.c:1008
#1  0x0044deee in simp_ref (t=0x80051998) at simp.c:104
#2  0x00453204 in simp_tree (t=0x80051998, context=0x0) at simp.c:752
#3  0x0046174e in tree_rewrite_aux (t=0x80051998, ctx=0x28aad0) at tree.c:1806
#4  0x00461352 in tree_rewrite_aux (t=0x80052ac8, ctx=0x28aad0) at tree.c:1763
#5  0x004613fb in tree_rewrite_aux (t=0x80052b18, ctx=0x28aad0) at tree.c:1768
#6  0x0046188f in tree_rewrite (t=0x80052b18, fn=0x453029 <simp_tree>, context=0x0) at tree.c:1828
#7  0x00453621 in simplify (top=0x80052b18) at simp.c:786
#8  0x004017e5 in analyse (argc=2, argv=0x28abf0) at nvc.c:119
#9  0x004044a8 in main (argc=2, argv=0x28abf0) at nvc.c:674
(gdb) n
1009       assert(item->tree != NULL);
(gdb) n
assertion "item->tree != NULL" failed: file "tree.c", line 1009, function: tree_ref

Program received signal SIGABRT, Aborted.
0x0028da18 in ?? ()
(gdb)
```