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_llvm/llvm-wrapper/LLVMWrapper.h

@@ -96,6 +96,7 @@ enum LLVMRustAttribute {
   AllocatedPointer = 38,
   AllocAlign = 39,
 #endif
+  SanitizeSafeStack = 40,
 };
 
 typedef struct OpaqueRustString *RustStringRef;

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -234,6 +234,8 @@ static Attribute::AttrKind fromRust(LLVMRustAttribute Kind) {
   case AllocAlign:
     return Attribute::AllocAlign;
 #endif
+  case SanitizeSafeStack:
+    return Attribute::SafeStack;
   }
   report_fatal_error("bad AttributeKind");
 }