Update LLVM global variable debug info API for 4.0

This teaches Rust about an LLVM 4.0 API change for creating debug info
for global variables.

This change was made in upstream LLVM patch https://reviews.llvm.org/D20147

This is almost 1:1 copy of how clang did it in http://reviews.llvm.org/D20415

src/rustllvm/RustWrapper.cpp

@@ -651,17 +651,32 @@ extern "C" LLVMRustMetadataRef LLVMRustDIBuilderCreateStaticVariable(
     bool isLocalToUnit,
     LLVMValueRef Val,
     LLVMRustMetadataRef Decl = NULL,
-    uint64_t AlignInBits = 0)
-{
-    return wrap(Builder->createGlobalVariable(
-        unwrapDI<DIDescriptor>(Context),
+    uint64_t AlignInBits = 0) {
+    Constant *InitVal = cast<Constant>(unwrap(Val));
+
+#if LLVM_VERSION_GE(4, 0)
+    llvm::DIExpression *InitExpr = nullptr;
+    if (llvm::ConstantInt *IntVal = llvm::dyn_cast<llvm::ConstantInt>(InitVal)) {
+      InitExpr = Builder->createConstantValueExpression(
+          IntVal->getValue().getSExtValue());
+    } else if (llvm::ConstantFP *FPVal = llvm::dyn_cast<llvm::ConstantFP>(InitVal)) {
+        InitExpr = Builder->createConstantValueExpression(
+                FPVal->getValueAPF().bitcastToAPInt().getZExtValue());
+    }
+#endif
+
+    return wrap(Builder->createGlobalVariable(unwrapDI<DIDescriptor>(Context),
         Name,
         LinkageName,
         unwrapDI<DIFile>(File),
         LineNo,
         unwrapDI<DIType>(Ty),
         isLocalToUnit,
-        cast<Constant>(unwrap(Val)),
+#if LLVM_VERSION_GE(4, 0)
+        InitExpr,
+#else
+        InitVal,
+#endif
         unwrapDIptr<MDNode>(Decl)
 #if LLVM_VERSION_GE(4, 0)
         , AlignInBits