Fix independent FE compilation: toSymbol remainings

Dsymbol::toSymbol was refactored into a visitor in commit
63e61f160784649cf2cbd1267aaf26e2a2468e73, but its traces
in gluestub.c remained.
Fix independent FE compilation: asmSemantic

AsmStatement::semantic was moved into a free function in commit
e1491c1f6911f095f4434f5a25e68e9330f52bde, but gluestub.c was
not correspondingly updated.
Fix independent FE compilation: buildClosure and retStyle

Both functions were turned into free functions in commit
022c70d12abee1bf9e1789567411ccd447160d62. Since there are
backend dependencies in toir.c, the patch add stubs in
gluestub.c.
Fix independent FE compilation: genhelpers

genhelpers was added to Module in commit
6d1882663b51dd72edeb62d78f2b23739713c3d5,
but not to gluestub.c.
Fix independent FE compilation: Nspace::toObjFile

Introduced in commit 1678b094d60559dbb33b8a2738696a96de31f99f,
but without a corresponding stub in gluestub.c.
Merge pull request #3823 from ltcmelo/fixFEcompile

Fix independent Front-End build