Add LLVM KCFI support to the Rust compiler

This commit adds LLVM Kernel Control Flow Integrity (KCFI) support to
the Rust compiler. It initially provides forward-edge control flow
protection for operating systems kernels for Rust-compiled code only by
aggregating function pointers in groups identified by their return and
parameter types. (See llvm/llvm-project@cff5bef.)

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) will be provided in later work as
part of this project by identifying C char and integer type uses at the
time types are encoded (see Type metadata in the design document in the
tracking issue #89653).

LLVM KCFI can be enabled with -Zsanitizer=kcfi.

Co-authored-by: bjorn3 <17426603+bjorn3@users.noreply.github.com>

src/type_.rs

@@ -300,4 +300,8 @@ impl<'gcc, 'tcx> TypeMembershipMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
         // Unsupported.
         self.context.new_rvalue_from_int(self.int_type, 0)
     }
+
+    fn set_kcfi_type_metadata(&self, _function: RValue<'gcc>, _kcfi_typeid: u32) {
+        // Unsupported.
+    }
 }