d: Fix regression returning from function with invariants [PR119139]

d: Fix regression returning from function with invariants [PR119139]

An optimization was added in GDC-12 which sets the TREE_READONLY flag on
all local variables with the storage class `const' assigned.  For some
reason, const is also being added by the front-end to `__result'
variables in non-virtual functions, which ends up getting wrong code by
the gimplify pass promoting the local to static storage.

A bug has been raised upstream, as this looks like an error in the AST.
For now, turn off setting TREE_READONLY on all result variables.

	PR d/119139

gcc/d/ChangeLog:

	* decl.cc (get_symbol_decl): Don't set TREE_READONLY for __result
	declarations.

gcc/testsuite/ChangeLog:

	* gdc.dg/pr119139.d: New test.

(cherry picked from commit 81582ca6cb692098c1bda7995aec46c6cbfbfcb3)