Add functions peekNextN(unsigned) and assignNewExpr(ArrayRef<uint64_t>) to...

Add functions peekNextN(unsigned) and assignNewExpr(ArrayRef<uint64_t>) to DIExpressionCursor (#71717)

This commit adds two functions to the DIExpressionCursor class.

`peekNextN(unsigned)` works like peekNext, but lets you peek the next
Nth element
    
`assignNewExpr(ArrayRef<uint64_t>)` lets you assign a new expression to
the same DIExpressionCursor object

This is part of a stack of patches, it comes after
https://github.com/llvm/llvm-project/pull/69768