Add "void debug (tree)"

Add "void debug (tree)"

Add "void debug (tree)" to support:

(gdb) call debug (expr)
 <parm_decl 0x7fffe9810bb0 f
    type <record_type 0x7fffe99cec78 c BLK
        size <integer_cst 0x7fffe98242d0 constant 256>
        unit-size <integer_cst 0x7fffe98243a8 constant 32>
        user align:256 warn_if_not_align:0 symtab:0 alias-set -1 canonical-type 0x7fffe99cebd0
        fields <field_decl 0x7fffe98318c0 a type <real_type 0x7fffe982a3f0 long double>
            XF x.c:2:15
            size <integer_cst 0x7fffe9802fa8 constant 128>
            unit-size <integer_cst 0x7fffe9802fc0 constant 16>
            align:128 warn_if_not_align:0 offset_align 128 decl_not_flexarray: 1
            offset <integer_cst 0x7fffe9802f90 constant 0>
            bit-offset <integer_cst 0x7fffe9802fd8 constant 0> context <record_type 0x7fffe99cebd0> chain <field_decl 0x7fffe9831960 b>>>
    used read BLK x.c:7:6 size <integer_cst 0x7fffe98242d0 256> unit-size <integer_cst 0x7fffe98243a8 32>
    align:256 warn_if_not_align:0 context <function_decl 0x7fffe99d2900 e> arg-type <record_type 0x7fffe99cec78 c>>
(gdb)

	PR debug/120849
	* print-tree.cc (debug): New.
	* print-tree.h (debug): Likewise.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>