src/comp.c: Use constructor expressions when possible.

src/comp.c: Use constructor expressions when possible.

* src/comp.c:
  - Add declarations for creating constructor/initializer expressions
  when supported.
  - (emit_coerce): Use a struct constructor expression to create a
  Lisp_Object value instead of creating a new local variable.
  - emit_limple_call_ref: Emit a single constructor expression for
  initializing tmp_arr.