Add cross-language LLVM CFI support to the Rust compiler

This commit adds cross-language LLVM Control Flow Integrity (CFI)
support to the Rust compiler by adding the
`-Zsanitizer-cfi-normalize-integers` option to be used with Clang
`-fsanitize-cfi-icall-normalize-integers` for normalizing integer types
(see https://reviews.llvm.org/D139395).

It provides forward-edge control flow protection for C or C++ and Rust
-compiled code "mixed binaries" (i.e., for when C or C++ and Rust
-compiled code share the same virtual address space). For more
information about LLVM CFI and cross-language LLVM CFI support for the
Rust compiler, see design document in the tracking issue #89653.

Cross-language LLVM CFI can be enabled with -Zsanitizer=cfi and
-Zsanitizer-cfi-normalize-integers, and requires proper (i.e.,
non-rustc) LTO (i.e., -Clinker-plugin-lto).

compiler/rustc_feature/src/builtin_attrs.rs

@@ -494,6 +494,12 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     // RFC 2397
     gated!(do_not_recommend, Normal, template!(Word), WarnFollowing, experimental!(do_not_recommend)),
 
+    // `#[cfi_encoding = ""]`
+    gated!(
+        cfi_encoding, Normal, template!(NameValueStr: "encoding"), ErrorPreceding,
+        experimental!(cfi_encoding)
+    ),
+
     // ==========================================================================
     // Internal attributes: Stability, deprecation, and unsafe:
     // ==========================================================================