[-Wunsafe-buffer-usage] Add Fixable for simple pointer dereference

This patch introduces PointerDereferenceGadget, a FixableGadget that emits
fixits to handle cases where a pointer that is identified as unsafe is
dereferenced. The current implementation only handles cases where the strategy
is to change the type of the raw pointer to std::span. The fixit for this
strategy is to fetch the first element from the corresponding span instance.

For example for the code below, the PointerDereferenceGadget emits a fixit for
S3 (S1, S2 are to be handled by other gadgets):

  S1: int *ptr = new int[10];
  S2: int val1 = ptr[k]; // Unsafe operation
  S3: int val2 = *ptr; => Fixit: int val2 = ptr[0];

Differential revision: https://reviews.llvm.org/D143206