Enable DWARF indexing to work with partial units

Loading debuginfo from a kernel with separate or re-combined debuginfo
results in the following failure:

Traceback (most recent call last):
  File "/home/jeffm/.local/bin/drgn", line 11, in <module>
    load_entry_point('drgn==0.0.3+39.gbf05d9bf', 'console_scripts', 'drgn')()
  File "/home/jeffm/.local/lib/python3.6/site-packages/drgn-0.0.3+39.gbf05d9bf-py3.6-linux-x86_64.egg/drgn/internal/cli.py", line 121, in main
    prog.load_debug_info(args.symbols, **args.default_symbols)
Exception: invalid DW_AT_decl_file 10

A typical kernel build results in DWARF information with
DW_TAG_compile_unit tags but the objcopy --only-keep-debug and eu-unstrip
commands use DW_TAG_partial_unit tags.  The DWARF indexer only handles
the former, so the file table isn't populated and we get the exception.

Fortunately, the two are more or less interchangeable. (See
http://dwarfstd.org/doc/Dwarf3.pdf, section E.2.3).  Accepting both for
indexing compile units results in a working session.

Author: jeffmahoney

libdrgn/dwarf_index.c

@@ -1410,7 +1410,8 @@ static struct drgn_error *read_abbrev_decl(const char **ptr, const char *end,
 		break;
 	}
 
-	if (should_index || tag == DW_TAG_compile_unit)
+	if (should_index || tag == DW_TAG_compile_unit ||
+	    tag == DW_TAG_partial_unit)
 		die_flags = tag;
 	else
 		die_flags = 0;
@@ -1470,7 +1471,8 @@ static struct drgn_error *read_abbrev_decl(const char **ptr, const char *end,
 				break;
 			}
 		} else if (name == DW_AT_stmt_list &&
-			   tag == DW_TAG_compile_unit &&
+			   (tag == DW_TAG_compile_unit ||
+			    tag == DW_TAG_partial_unit) &&
 			   cu->sections[SECTION_DEBUG_LINE]) {
 			switch (form) {
 			case DW_FORM_data4:
@@ -2100,7 +2102,7 @@ static struct drgn_error *index_cu(struct drgn_dwarf_index *dindex,
 		}
 
 		tag = die.flags & TAG_MASK;
-		if (tag == DW_TAG_compile_unit) {
+		if (tag == DW_TAG_compile_unit || tag == DW_TAG_partial_unit) {
 			if (depth == 0 && die.stmt_list != SIZE_MAX &&
 			    (err = read_file_name_table(dindex, cu,
 							die.stmt_list,