This patch does two things.
1. Previously, when checking driver arguments, we emitted an error for
unsupported values of `-mbranch-protection` when using pauthtest ABI.
The reason for that was ptrauth-returns being enabled as part of
pauthtest. This patch changes the check against pauthtest to a check
against ptrauth-returns.
2. Similarly, check against values of the following function attribute
which are unsupported with ptrauth-returns:
`__attribute__((target("branch-protection=XXX`. Note that existing
`validateBranchProtection` function is used, and current behavior is to
ignore the unsupported attribute value, so no error is emitted.
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