PAC/BTI-related codegen in the AArch64 backend is controlled by a set
a set of LLVM IR function attributes, added to the function by Clang, based
based on command-line options and GCC-style function attributes. However,
However, functions, generated in the LLVM middle end (for example,
`asan.module.ctor` or `__llvm_gcov_write_out`) do not get any attributes
and the backend incorrectly does not do any PAC/BTI code generation.
This patch record the default state of PAC/BTI codegen in a set of
LLVM IR module-level attributes, based on command-line options:
* "sign-return-address", if present, with non-zero value means generate code to sign return
addresses with defaults begin using A-key on functions which spill LR sign return addresses (PAC-RET), zero value means disable PAC-RET.
* "sign-return-address-all", if present, extends the return address signing,with non-zero value means enable PAC-RET
enabled by "sign-return-address" to for all functions, which do not spill LR as well.zero value means enable PAC-RET only for functions,
* "sign-return-address-with-bkey", if present, modified the default, which spill LR.
established by "sign-return-address" to using to B-key
* "sign-return-address-with-bkey", with non-zero value means use
B-key for signing, zero value mean use A-key.
This set of attributes are always added for AArch64 targets (as
opposed, for example, to interpreting a missing attribute as having a
value 0) in order to be able to check for conflicts when combining
module attributed during LTO.
Module-level attributes are overridden by function level attributes.
All the decision making about whether to not to generate PAC and/or
BTI code is factored out into `AArch64FunctionInfo`, there shouldn't be
any places left, other than `AArch64FunctionInfo`, which directly
examine PAC/BTI attributes, except `AArch64AsmPrinter.cpp`, which
is/will-be handled by a separate patch.