[Clang] Add -fwrapv-pointer flag (#122486)

GCC supports three flags related to overflow behavior:
* `-fwrapv`: Makes signed integer overflow well-defined.
* `-fwrapv-pointer`: Makes pointer over

[Clang] Add -fwrapv-pointer flag (#122486)

GCC supports three flags related to overflow behavior:
* `-fwrapv`: Makes signed integer overflow well-defined.
* `-fwrapv-pointer`: Makes pointer overflow well-defined.
* `-fno-strict-overflow`: Implies `-fwrapv -fwrapv-pointer`, making both
signed integer overflow and pointer overflow well-defined.

Clang currently only supports `-fno-strict-overflow` and `-fwrapv`, but
not `-fwrapv-pointer`.

This PR proposes to introduce `-fwrapv-pointer` and adjust the semantics
of `-fwrapv` to match GCC.

This allows signed integer overflow and pointer overflow to be
controlled independently, while `-fno-strict-overflow` still exists to
control both at the same time (and that option is consistent across GCC
and Clang).

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