d: Forbid taking the address of an intrinsic with no implementation

This code fails to link:

    import core.math;
    real function(real) fn = &sin;

However, when called directly, the D intrinsic `sin()' is expanded by
the front-end into the GCC built-in `__builtin_sin()'.  This has been
fixed to now also expand the function when a reference is taken.

As there are D intrinsics and GCC built-ins that don't have a fallback
implementation, raise an error if taking the address is not possible.

gcc/d/ChangeLog:

	* d-tree.h (intrinsic_code): Update define for DEF_D_INTRINSIC.
	(maybe_reject_intrinsic): New prototype.
	* expr.cc (ExprVisitor::visit (SymOffExp *)): Call
	maybe_reject_intrinsic.
	* intrinsics.cc (intrinsic_decl): Add fallback field.
	(intrinsic_decls): Update define for DEF_D_INTRINSIC.
	(maybe_reject_intrinsic): New function.
	* intrinsics.def (DEF_D_LIB_BUILTIN): Update.
	(DEF_CTFE_BUILTIN): Update.
	(INTRINSIC_BSF): Declare as library builtin.
	(INTRINSIC_BSR): Likewise.
	(INTRINSIC_BT): Likewise.
	(INTRINSIC_BSF64): Likewise.
	(INTRINSIC_BSR64): Likewise.
	(INTRINSIC_BT64): Likewise.
	(INTRINSIC_POPCNT32): Likewise.
	(INTRINSIC_POPCNT64): Likewise.
	(INTRINSIC_ROL): Likewise.
	(INTRINSIC_ROL_TIARG): Likewise.
	(INTRINSIC_ROR): Likewise.
	(INTRINSIC_ROR_TIARG): Likewise.
	(INTRINSIC_ADDS): Likewise.
	(INTRINSIC_ADDSL): Likewise.
	(INTRINSIC_ADDU): Likewise.
	(INTRINSIC_ADDUL): Likewise.
	(INTRINSIC_SUBS): Likewise.
	(INTRINSIC_SUBSL): Likewise.
	(INTRINSIC_SUBU): Likewise.
	(INTRINSIC_SUBUL): Likewise.
	(INTRINSIC_MULS): Likewise.
	(INTRINSIC_MULSL): Likewise.
	(INTRINSIC_MULU): Likewise.
	(INTRINSIC_MULUI): Likewise.
	(INTRINSIC_MULUL): Likewise.
	(INTRINSIC_NEGS): Likewise.
	(INTRINSIC_NEGSL): Likewise.
	(INTRINSIC_TOPRECF): Likewise.
	(INTRINSIC_TOPREC): Likewise.
	(INTRINSIC_TOPRECL): Likewise.

gcc/testsuite/ChangeLog:

	* gdc.dg/builtins_reject.d: New test.
	* gdc.dg/intrinsics_reject.d: New test.

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