Add support for nested composite locations

Add support for nested composite locations

After allowing a location description to be placed on a DWARF stack,
in an effort to achieve a full composability of the DWARF expression,
it is necessary to enable forming of a nested composite location
descriptions.

To be able do this, a new operation DW_OP_LLVM_piece_end needs to be
introduced, along with some additional rules on the way how the
composite location description is formed using the existing DW_OP_piece
and DW_OP_bit_piece operations. These new rules are fully compatible
with the composite forming rules from the DWARF 5 standard.

More details on the new operation and added rules can be found here:

https://llvm.org/docs/AMDGPUDwarfExtensionsForHeterogeneousDebugging.html

gdb/ChangeLog:

	* compile/compile-loc2c.c (compute_stack_depth_worker): Add new
	DW_OP_LLVM_piece_end operation support.
	* dwarf2/expr.c (class dwarf_value): Add copy constructor.
	(class dwarf_location): Add copy constructor.
	(class dwarf_undefined): Add copy constructor.
	(class dwarf_memory): Add copy constructor.
	(class dwarf_register): Add copy constructor.
	(class dwarf_implicit): Add copy constructor.
	(class dwarf_implicit_pointer): Add copy constructor.
	(class dwarf_composite): Add copy constructor.
	(read_from_location): Add composite completed check.
	(write_to_location): Add composite completed check.
	(read_value_contents_from_location): New function.
	(dwarf_entry_factory::copy_entry): New method.
	(rw_closure_value): Now calls read_value_contents_from_location
	function.
	(dwarf_expr_context::add_piece): Use new composite forming
	rules.
	(dwarf_expr_context::execute_stack_op): Add new
	DW_OP_LLVM_piece_end operation support.
	* dwarf2/loc.c (dwarf2_get_symbol_read_needs): Add new
	DW_OP_LLVM_piece_end operation support.

include/ChangeLog:

	* dwarf2.def (DW_OP_DUP): Add new DW_OP_LLVM_piece_end
          enumeration.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/dw2-llvm-piece-end.exp: New test.

Change-Id: Ib0b25e5de3f23df89d7d9e86aad56029c7d173df