[-Wunsafe-buffer-usage] Generate fix-it for local variable declarations

Use clang fix-its to transform declarations of local variables, which are used for buffer access , to be of std::span type.

We placed a few limitations to keep the solution simple:
- it only transforms local variable declarations (no parameter declaration);
- it only considers single level pointers, i.e., pointers of type T * regardless of whether T is again a pointer;
- it only transforms to std::span types (no std::array, or std::span::iterator, or ...);
- it can only transform a VarDecl that belongs to a DeclStmt whose has a single child.

One of the purposes of keeping this patch simple enough is to first
evaluate if fix-it is an appropriate approach to do the
transformation.

Reviewed by: NoQ, jkorous

Differential revision: https://reviews.llvm.org/D139737

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