[-Wunsafe-buffer-usage] Add a Fixable for pointer pre-increment

For a pointer type expression `e` of the form `++DRE`, if `e` is under
an Unspecified Pointer Context (UPC) and `DRE` is suppose to be
transformed to have std:span type, we generate fix-its that transform `e` to
`(DRE = DRE.subspan(1)).data()`.

For reference, `e` is in an UPC if `e` is
- an argument of a function call (except the callee has [[unsafe_buffer_usage]] attribute), or
- the operand of a cast-to-(Integer or Boolean) operation; or
- the operand of a pointer subtraction operation; or
- the operand of a pointer comparison operation;

We may extend the definition of UPC by adding more cases later.

Reviewed by: NoQ (Artem Dergachev)

Differential revision: https://reviews.llvm.org/D144304

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