[Instruction] Add dropLocation and updateLocationAfterHoist helpers

[Instruction] Add dropLocation and updateLocationAfterHoist helpers

Introduce a helper which can be used to update the debug location of an
Instruction after the instruction is hoisted. This can be used to safely
drop a source location as recommended by the docs.

For more context, see the discussion in https://reviews.llvm.org/D60913.

Differential Revision: https://reviews.llvm.org/D85670