Fixes #7563

Inside check_str_format_call method, it checks if expression of format method call is an Enum member and it takes Literal value of that Enum member to check the format call arguments, if so.

Diff from mypy_primer, showing the effect of this PR on open source code:

discord.py (https://github.com/Rapptz/discord.py): typechecking got 1.09x faster (194.1s -> 178.0s)
(Performance measurements are based on a single noisy sample)

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