Add a module Hash in the bitcode and the combined index, implementing a kind of "build-id"

This is intended to be used for ThinLTO incremental build.

Differential Revision: http://reviews.llvm.org/D18213

This is a recommit of r265095 after fixing the Windows issues.

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265111 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/ModuleSummaryIndex.cpp

@@ -37,9 +37,11 @@ void ModuleSummaryIndex::mergeFrom(std::unique_ptr<ModuleSummaryIndex> Other,
 
     // Add the module path string ref for this module if we haven't already
     // saved a reference to it.
-    if (ModPath.empty())
-      ModPath = addModulePath(Info->summary()->modulePath(), NextModuleId);
-    else
+    if (ModPath.empty()) {
+      auto Path = Info->summary()->modulePath();
+      ModPath = addModulePath(Path, NextModuleId, Other->getModuleHash(Path))
+                    ->first();
+    } else
       assert(ModPath == Info->summary()->modulePath() &&
              "Each module in the combined map should have a unique ID");