Fix DWARF disassembly of DW_OP_const_type

Fix DWARF disassembly of DW_OP_const_type

While debugging another issue, I noticed that disassembling a DWARF
expression using DW_OP_const_type did not work.
disassemble_dwarf_expression was not properly decoding this operation.

This patch fixes the problem.  Tested by re-debugging gdb.

I didn't write a test case because that seemed like overkill for
what's essentially a maintainer's helper.

The expression evaluator does decode this properly, so no other change
was needed.

gdb/ChangeLog
2020-04-03  Tom Tromey  <tromey@adacore.com>

	* dwarf2/loc.c (disassemble_dwarf_expression) <DW_OP_const_type>:
	Read constant block.