[-Wunsafe-buffer-usage] FixableGadget for handling stand alone pointers under UPC

[-Wunsafe-buffer-usage] FixableGadget for handling stand alone pointers under UPC

This patch introduces UPCStandalonePointerGadget, a FixableGadget that emits fixits to
handle cases where a pointer identified as unsafe is simply referenced. An example of
such a case is when the pointer is input as an argument to a method call, where we can
not change the type of the argument. For cases where the strategy for the unsafe pointer is
to use std::span, the idea is to extract the underlying pointer by invoking the "data()"
method on the span instance.

For example, the gadget emits a fixit for S3, where S1, S2 are handled by other gadgets:
  S1: int *ptr = new int[10];
  S2: int val1 = ptr[k]; // Unsafe operation on ptr
  S3: foo(ptr); // Some method that accepts raw pointer => FIXIT: foo(ptr.data());

Reviewed by: NoQ, ziqingluo-90, jkorous

Differential revision: https://reviews.llvm.org/D143676