d: Add `@register' attribute to compiler and library.

d: Add `@register' attribute to compiler and library.

The `@register` attribute specifies that a local or `__gshared` variable
is to be given a register storage-class in the C sense of the term, and
will be placed into a register named `registerName`.

The variable needs to boiled down to a data type that fits the target
register.  It also cannot have either thread-local or `extern` storage.
It is an error to take the address of a register variable.

	PR d/105413

gcc/d/ChangeLog:

	* d-attribs.cc (d_handle_register_attribute): New function.
	(d_langhook_attribute_table): Add register attribute.
	* d-codegen.cc (d_mark_addressable): Error if taken address of
	register variable.
	(build_frame_type): Error if register variable has non-local
	references.
	* d-tree.h (d_mark_addressable): Add complain parameter.
	* decl.cc (get_symbol_decl): Mark register varibles DECL_REGISTER.
	Error when register variable declared thread-local or extern.
	* expr.cc (ExprVisitor::visit (IndexExp *)): Don't complain about
	marking register vectors as addressable in an ARRAY_REF.

libphobos/ChangeLog:

	* libdruntime/gcc/attributes.d (register): Define.

gcc/testsuite/ChangeLog:

	* gdc.dg/attr_register1.d: New test.
	* gdc.dg/attr_register2.d: New test.
	* gdc.dg/attr_register3.d: New test.