Add singlethreaded fence intrinsics.

These new intrinsics are comparable to `atomic_signal_fence` in C++,
ensuring the compiler will not reorder memory accesses across the
barrier, nor will it emit any machine instructions for it.

Closes #24118, implementing RFC 888.

src/librustc_llvm/lib.rs

@@ -43,6 +43,7 @@ pub use self::RealPredicate::*;
 pub use self::TypeKind::*;
 pub use self::AtomicBinOp::*;
 pub use self::AtomicOrdering::*;
+pub use self::SynchronizationScope::*;
 pub use self::FileType::*;
 pub use self::MetadataType::*;
 pub use self::AsmDialect::*;
@@ -361,6 +362,13 @@ pub enum AtomicOrdering {
     SequentiallyConsistent = 7
 }
 
+#[repr(C)]
+#[derive(Copy, Clone)]
+pub enum SynchronizationScope {
+    SingleThread = 0,
+    CrossThread = 1
+}
+
 // Consts for the LLVMCodeGenFileType type (in include/llvm/c/TargetMachine.h)
 #[repr(C)]
 #[derive(Copy, Clone)]
@@ -1534,7 +1542,9 @@ extern {
                               SingleThreaded: Bool)
                               -> ValueRef;
 
-    pub fn LLVMBuildAtomicFence(B: BuilderRef, Order: AtomicOrdering);
+    pub fn LLVMBuildAtomicFence(B: BuilderRef,
+                                Order: AtomicOrdering,
+                                Scope: SynchronizationScope);
 
 
     /* Selected entries from the downcasts. */