llvm-ar: Darwin archive format fixes.

* Support writing the DARWIN64 symbol table format.

* In darwin archives, emit a symbol table whenever requested, even
  when there are no members, as the apple linker will abort if given
  an archive without a symbol table.

Added tests for same, and also simplified and moved the GNU 64-bit
symbol table test into archive-symtab.test.

llvm-svn: 344183
diff --git a/llvm/test/Object/archive-format.test b/llvm/test/Object/archive-format.test
index 219fc7f..b1ae411 100644
--- a/llvm/test/Object/archive-format.test
+++ b/llvm/test/Object/archive-format.test
@@ -38,7 +38,7 @@
 BSD-NEXT: 0123456789abcdefzed.
 
 RUN: rm -f %t.a
-RUN: llvm-ar --format=darwin rc %t.a 0123456789abcde 0123456789abcdef
+RUN: llvm-ar --format=darwin rcS %t.a 0123456789abcde 0123456789abcdef
 RUN: cat %t.a | FileCheck -strict-whitespace --check-prefix=DARWIN %s
 
 DARWIN:      !<arch>