[AArch64][PAC] Use enum to describe LR signing condition (NFC) (#168548)

Express the condition of signing the return address in a function using
an `enum class` instead of a pair of `bool`s. Define `enum class
SignReturnAddress` with the values corresponding to the three possible
modes that can be requested via "sign-return-address" function
attribute.

Previously, there were two overloads of `shouldSignReturnAddress`
accepting either `const MachineFunction &` or `bool` argument. Due to
pointer-to-bool conversion, when `shouldSignReturnAddress` was
incorrectly called with `const MachineFunction *` argument, the latter
overload was used instead of reporting a compile-time error.

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