Make Rustc build with LLVM trunk.

Author: vext01

src/rustllvm/RustWrapper.cpp

@@ -793,10 +793,17 @@ extern "C" LLVMMetadataRef LLVMRustDIBuilderCreateStaticVariable(
         FPVal->getValueAPF().bitcastToAPInt().getZExtValue());
   }
 
+#if LLVM_VERSION_GE(8, 0)
+  llvm::DIGlobalVariableExpression *VarExpr = Builder->createGlobalVariableExpression(
+      unwrapDI<DIDescriptor>(Context), Name, LinkageName,
+      unwrapDI<DIFile>(File), LineNo, unwrapDI<DIType>(Ty), IsLocalToUnit,
+      InitExpr, unwrapDIPtr<MDNode>(Decl), nullptr, AlignInBits);
+#else
   llvm::DIGlobalVariableExpression *VarExpr = Builder->createGlobalVariableExpression(
       unwrapDI<DIDescriptor>(Context), Name, LinkageName,
       unwrapDI<DIFile>(File), LineNo, unwrapDI<DIType>(Ty), IsLocalToUnit,
       InitExpr, unwrapDIPtr<MDNode>(Decl), AlignInBits);
+#endif
 
   InitVal->setMetadata("dbg", VarExpr);