d: Fix accesses of immutable arrays using constant index still bounds checked

d: Fix accesses of immutable arrays using constant index still bounds checked

Starts setting TREE_READONLY against specific kinds of VAR_DECLs, so
that the middle-end/optimization passes can more aggressively constant
fold D code that makes use of `immutable' or `const'.

	PR d/110514

gcc/d/ChangeLog:

	* decl.cc (get_symbol_decl): Set TREE_READONLY on certain kinds of
	const and immutable variables.
	* expr.cc (ExprVisitor::visit (ArrayLiteralExp *)): Set TREE_READONLY
	on immutable dynamic array literals.

gcc/testsuite/ChangeLog:

	* gdc.dg/pr110514a.d: New test.
	* gdc.dg/pr110514b.d: New test.
	* gdc.dg/pr110514c.d: New test.
	* gdc.dg/pr110514d.d: New test.