Add SafeStack support to rustc

Adds support for LLVM [SafeStack] which provides backward edge control
flow protection by separating the stack into two parts: data which is
only accessed in provable safe ways is allocated on the normal stack
(the "safe stack") and all other data is placed in a separate allocation
(the "unsafe stack").

SafeStack support is enabled by passing `-Zsanitizer=safestack`.

[SafeStack]: https://clang.llvm.org/docs/SafeStack.html

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -88,6 +88,9 @@ pub fn sanitize_attrs<'ll>(
 
         attrs.push(llvm::AttributeKind::SanitizeMemTag.create_attr(cx.llcx));
     }
+    if enabled.contains(SanitizerSet::SAFESTACK) {
+        attrs.push(llvm::AttributeKind::SanitizeSafeStack.create_attr(cx.llcx));
+    }
     attrs
 }