IRGen: add sppreq instead of sspstrong for functions which need stack protection.

Now that Swift decides whether or not we need stack protection we should force LLVM to emit stack protection when we request it.

Author: eeckstein

lib/IRGen/IRGenModule.cpp

@@ -1265,7 +1265,7 @@ void IRGenModule::constructInitialFnAttributes(
     Attrs.removeAttribute(llvm::Attribute::OptimizeForSize);
   }
   if (stackProtector == StackProtectorMode::StackProtector) {
-    Attrs.addAttribute(llvm::Attribute::StackProtectStrong);
+    Attrs.addAttribute(llvm::Attribute::StackProtectReq);
     Attrs.addAttribute("stack-protector-buffer-size", llvm::utostr(8));
   }
 }

test/IRGen/stack_protector.swift

@@ -21,5 +21,5 @@ public func noStackProtector() {
     g(x)
     g(x) // avoid function merging by calling `g` two times
 }
+// CHECK: [[SSPATTRS]] = { sspreq {{.*}}"stack-protector-buffer-size"="8"
 
-// CHECK: [[SSPATTRS]] = { sspstrong {{.*}}"stack-protector-buffer-size"="8"