[compiler-rt] Add 'ptrauth' lit feature

[delcypher]

@yln What do you want the ptrauth feature to mean? Right now it means the target is arm64e. I would expect to mean that the toolchain and the target support ptrauth. Right now the patch only checks the target. The patch doesn't check if the toolchain supports it.

What happens if you use a freshly built clang (from llvm-project from llvm.org, not from Swift) for this test? Does it pass? If so I guess my point doesn't matter.

If it doesn't then we might want to a check (probably at CMake configure time) if the toolchain supports ptrauth.

compiler-rt/test/lit.common.cfg.py

 config.substitutions.append( ("%expect_crash ", config.expect_crash) )
 
 target_arch = getattr(config, 'target_arch', None)
 if target_arch:
   config.available_features.add(target_arch + '-target-arch')
   if target_arch in ['x86_64', 'i386']:
     config.available_features.add('x86-target-arch')
   config.available_features.add(target_arch + '-' + config.host_os.lower())
+  if target_arch in ['arm64e']:
+    config.available_features.add('ptrauth')
 
 compiler_rt_debug = getattr(config, 'compiler_rt_debug', False)
 if not compiler_rt_debug:
   config.available_features.add('compiler-rt-optimized')
 
 libdispatch = getattr(config, 'compiler_rt_intercept_libdispatch', False)
 if libdispatch:
   config.available_features.add('libdispatch')

compiler-rt/test/ubsan/TestCases/TypeCheck/vptr-ptrauth-unauthenticated.cpp

 // Test that we don't crash for vtable pointers with an invalid ptrauth
 // signature which includes unauthenticated vtable pointers.
 
 // RUN: %clangxx -frtti -fsanitize=vptr -fno-sanitize-recover=vptr -g %s -O3 -o %t
 // RUN: not %run %t 2>&1 | FileCheck %s
 
-// TODO(yln): introduce 'ptrauth' lit feature
-// REQUIRES: stable-runtime, cxxabi, arch=arm64e
+// REQUIRES: stable-runtime, cxxabi, ptrauth
 
 #include <typeinfo>
 #include <ptrauth.h>
 
 struct S {
   S() {}
   ~S() {}
   virtual int v() { return 0; }