The line itself contains a few literary devices:
* Metaphor: "Heat the sun" is a metaphor for the heat of life, of worldly cares and passions. It is a figurative representation of the troubles and anxieties that Lear has faced.
* Personification: The sun is personified by being given the action "heat". This gives the sun a more active role and makes it feel like a source of worry and tension.
* Alliteration: The repetition of the "h" sound in "heat" and "the" emphasizes the feeling of warmth and intensity, creating a sense of comforting heat that contrasts with the cold reality of death.
However, the most significant literary device in this phrase, and in the song as a whole, is imagery. It conjures up a sense of peace, comfort, and release from the burdens of life. The idea of no longer fearing the "heat the sun" evokes a sense of serenity and transcendence.
The song is a beautiful example of Shakespeare's use of language to create a powerful emotional impact, and the line "Fear no more heat the sun" is a key part of that.