ELFObjectWriter: deduplicate suffices in strtab

We already do this for shstrtab, so might as well do it for strtab. This
extracts the string table building code into a separate class. The idea
is to use it for other object formats too.

I mostly wanted to do this for the general principle, but it does save a
little bit on object file size. I tried this on a clang bootstrap and
saved 0.54% on the sum of object file sizes (1.14 MB out of 212 MB for
a release build).

Differential Revision: http://reviews.llvm.org/D3533

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207670 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/Object/StringTableBuilderTest.cpp

@@ -0,0 +1,40 @@
+//===----------- StringTableBuilderTest.cpp -------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "gtest/gtest.h"
+#include "llvm/Object/StringTableBuilder.h"
+#include <string>
+
+using namespace llvm;
+
+namespace {
+
+TEST(StringTableBuilderTest, Basic) {
+  StringTableBuilder B;
+
+  B.add("foo");
+  B.add("bar");
+  B.add("foobar");
+
+  B.finalize();
+
+  std::string Expected;
+  Expected += '\x00';
+  Expected += "foobar";
+  Expected += '\x00';
+  Expected += "foo";
+  Expected += '\x00';
+
+  EXPECT_EQ(Expected, B.data());
+  EXPECT_EQ(1U, B.getOffset("foobar"));
+  EXPECT_EQ(4U, B.getOffset("bar"));
+  EXPECT_EQ(8U, B.getOffset("foo"));
+}
+
+}