core.demangle.mangleFunc for different linkage types

- error out for extern(C++)
- preserve plain names for extern(C), extern(Pascal) and extern(Windows)
add static to kill pointless allocation
Merge pull request #624 from dawgfoto/mangleFunc

core.demangle.mangleFunc for different linkage types
Merge pull request #625 from adamdruppe/bug_10323

add static to kill pointless allocation