Add support for gnu archives with a string table and no symtab.

While there, use early returns to reduce nesting.

llvm-svn: 185547
diff --git a/llvm/test/Object/Inputs/archive-test.a-gnu-no-symtab b/llvm/test/Object/Inputs/archive-test.a-gnu-no-symtab
new file mode 100644
index 0000000..0a5b237
--- /dev/null
+++ b/llvm/test/Object/Inputs/archive-test.a-gnu-no-symtab
@@ -0,0 +1,5 @@
+!<arch>
+//                                              24        `
+a-very-long-file-name/
+
+/0              1372864788  1000  1000  100664  0         `