d: Merge upstream dmd 4d07f22f2, druntime f89da313, phobos d46814c86.

D front-end changes:

    - `scope' semantics are now enforced in `@safe' code on pointers to
      stack memory, but only as deprecation warnings.
    - Overriding virtual functions are now marked with the `override'
      and `final' in the generated headers of `-fdump-c++-spec='.
    - `-fpreview=fiximmmutableconv` has been added that disallows
      implicitly converting a return value with indirections to
      immutable if it determines the result must be unique.

D runtime changes:

    - Posix (excluding Darwin): Switch default GC signals from SIGUSR1/2
      to SIGRTMIN/SIGRTMIN+1

Phobos changes:

    - Import latest bug fixes to mainline.

gcc/d/ChangeLog:

	* dmd/MERGE: Merge upstream dmd 4d07f22f2
	* d-lang.cc (d_handle_option): Handle OPT_fpreview_fiximmutableconv.
	* lang.opt (fpreview=fiximmutableconv): New option.
	* runtime.def (ARRAYAPPENDT): Remove.

libphobos/ChangeLog:

	* libdruntime/MERGE: Merge upstream druntime f89da313.
	* src/MERGE: Merge upstream phobos d46814c86.

Signed-off-by: Iain Buclaw <ibuclaw@gdcproject.org>