Introduce dwarf2_per_objfile::obstack

Currently much of the DWARF-related data is stored on the objfile
obstack.  This prevents sharing this data across objfiles, so this patch
adds a new obstack to dwarf2_per_objfile.  Note that the
dwarf2_per_objfile type is going to become "dwarf2_per_bfd" in a
subsequent patch, which is indeed going to be shared between objfiles.

One way to check whether this is correct is to look at the remaining
uses of objfile_obstack in the DWARF code and note that they all
appear in the "full CU" code paths.

The converse -- storing per-objfile data on the shared obstack -- is
not good, but it is just a memory leak, not a potential
use-after-free.  Double-checking this would also be useful, though.

gdb/ChangeLog:

	* dwarf2/read.h (struct dwarf2_per_objfile) <obstack>: New
	member.
	* dwarf2/read.c (delete_file_name_entry): Fix comment.
	(create_cu_from_index_list)
	(create_signatured_type_table_from_index)
	(create_signatured_type_table_from_debug_names)
	(dw2_get_file_names_reader, dwarf2_initialize_objfile)
	(dwarf2_create_include_psymtab)
	(create_debug_type_hash_table, add_type_unit)
	(create_type_unit_group, read_comp_units_from_section)
	(dwarf2_compute_name, create_cus_hash_table)
	(create_dwp_hash_table, create_dwo_unit_in_dwp_v1)
	(create_dwo_unit_in_dwp_v2, open_and_init_dwp_file): Use new
	obstack.
	(dw2_get_real_path): Likewise.  Change argument to
	dwarf2_per_objfile.

Change-Id: Icdec7be7c4d9f33d1dce4f807284f3077f7d3f03

gdb/dwarf2/read.h

@@ -126,6 +126,11 @@ private:
 			const dwarf2_debug_sections &names);
 
 public:
+  /* Objects that can be shared across objfiles are stored in this
+     obstack or on the psymtab obstack, while objects that are
+     objfile-specific are stored on the objfile obstack.  */
+  auto_obstack obstack;
+
   dwarf2_section_info info {};
   dwarf2_section_info abbrev {};
   dwarf2_section_info line {};