decl.c (gnat_to_gnu_entity): Do not turn Ada Pure into GCC const, now implicitely implying nothrow as well.

        ada/
        * decl.c (gnat_to_gnu_entity) <case E_Function>: Do not turn Ada
        Pure into GCC const, now implicitely implying nothrow as well.

        testsuite/
        * gnat.dg/raise_from_pure.ad[bs],
        * gnat.dg/wrap_raise_from_pure.ad[bs]: Support for ...
        * gnat.dg/test_raise_from_pure.adb: New test.

From-SVN: r138509