[ARM][clang] Add back branch protection tests

[stuij]

Summary

When committing the PACBTI-M frontend support
patch (https://reviews.llvm.org/D112421), the tests in
arm-invalid-branch-protection.c were failing on certain test setups, so it was
removed to make the llvm test suite pass. The fix is to require
arm-registered-target.

This patch is part of a series that adds support for the PACBTI-M extension of
the Armv8.1-M architecture, as detailed here:

https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/armv8-1-m-pointer-authentication-and-branch-target-identification-extension

The PACBTI-M specification can be found in the Armv8-M Architecture Reference
Manual:

https://developer.arm.com/documentation/ddi0553/latest

[erichkeane]

I'm quite sure this is the right fix, the 'requires' clause missing is, I believe, the problems that caused us to revert this test.

clang/test/Frontend/arm-invalid-branch-protection.c

+// REQUIRES: arm-registered-target
+// RUN: %clang -target arm-arm-none-eabi -mbranch-protection=pac-ret+b-key -c %s -o /dev/null 2>&1 | FileCheck %s
+// RUN: %clang -target arm-arm-none-eabi -mbranch-protection=pac-ret+b-key+leaf -c %s -o /dev/null 2>&1 | FileCheck %s
+// RUN: %clang -target arm-arm-none-eabi -mbranch-protection=bti+pac-ret+b-key -c %s -o /dev/null 2>&1 | FileCheck %s
+// RUN: %clang -target arm-arm-none-eabi -mbranch-protection=bti+pac-ret+b-key+leaf -c %s -o /dev/null 2>&1 | FileCheck %s
+
+// CHECK: warning: invalid branch protection option 'b-key' in '-mbranch-protection={{[a-z+-]*}}' [-Wbranch-protection]