PR16974: wrap if, try and for tmpvar decls in unions

PR16974: wrap if, try and for tmpvar decls in unions

translate.cxx: in c_tmpcounter::visit_block, the tmpvar declarations
in the generated c code are wrapped in a union, with each
block->statements[i] wrapped in a struct, to reduce context memory
allocation. following ::visit_block, the tmpvar declarations in
::visit_if_statement, ::visit_try_block and ::visit_for_loop are
wrapped in a union. since this causes quite a lot of nesting, comments
are added to the generated c code to indicate the source file and
line the struct/union tmpvars correspond to.