Add support for Control Flow Guard on Windows.

This patch enables rustc to emit the required LLVM module flags to enable Control Flow Guard metadata (cfguard=1) or metadata and checks (cfguard=2). The LLVM module flags are ignored on unsupported targets and operating systems.

src/test/codegen/cfguard_checks.rs

@@ -0,0 +1,10 @@
+// compile-flags: -Z control_flow_guard=checks
+
+#![crate_type = "lib"]
+
+// A basic test function.
+pub fn test() {
+}
+
+// Ensure the module flag cfguard=2 is present
+// CHECK: !"cfguard", i32 2

src/test/codegen/cfguard_disabled.rs

@@ -0,0 +1,10 @@
+// compile-flags: -Z control_flow_guard=disabled
+
+#![crate_type = "lib"]
+
+// A basic test function.
+pub fn test() {
+}
+
+// Ensure the module flag cfguard is not present
+// CHECK-NOT: !"cfguard"

src/test/codegen/cfguard_nochecks.rs

@@ -0,0 +1,10 @@
+// compile-flags: -Z control_flow_guard=nochecks
+
+#![crate_type = "lib"]
+
+// A basic test function.
+pub fn test() {
+}
+
+// Ensure the module flag cfguard=1 is present
+// CHECK: !"cfguard", i32 1