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/llvm/ffi.rs

@@ -196,6 +196,7 @@ pub enum AttributeKind {
     AllocSize = 37,
     AllocatedPointer = 38,
     AllocAlign = 39,
+    SanitizeSafeStack = 40,
 }
 
 /// LLVMIntPredicate