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

test/Bitcode/Inputs/module_hash.ll

@@ -0,0 +1,4 @@
+; Needs a function for the combined index to be populated
+define void @bar() {
+    ret void
+}