tree-pretty-print: delimit TREE_VEC with braces

This makes the generic pretty printer print braces around a TREE_VEC,
like we do for CONSTRUCTOR.  This should improve readability of nested
TREE_VECs in particular.

gcc/ChangeLog:

	* tree-pretty-print.cc (dump_generic_node) <case TREE_VEC>:
	Delimit output with braces.