c++: Update const_decl handling after r15-7259 [PR118673].

Objective-C++ uses CONST_DECLs to hold constant string objects
these should also be treated as mergable lvalues.

	PR c++/118673

gcc/cp/ChangeLog:

	* tree.cc (lvalue_kind): Mark CONST_DECLs as mergable
	when they are also TREE_STATIC.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>