Fix expect_fun_call producing invalid suggestions

[sethdusek]

Previously expect_fun_call would too eagerly convert cases like foo.expect(if | block | match) into foo.unwrap_or_else. Additionally, it would also add to_string() even though the argument is being passed into format!() which can accept a &str. I also discovered some other cases where this lint would either produce invalid results, or be triggered unnecessarily:

• Clippy would suggest changing expect to unwrap_or_else even if the expression inside expect contains a return statement
• opt.expect(const_fn()) no longer triggers the lint
• The lint would always add braces to the closure body, even if the body of expect is a single expression
• opt.expect({"literal"}) used to get turned into opt.unwrap_or_else(|| panic!("{}", {"literal"}.to_string()))

Fixes #15056

changelog: [expect_fun_call]: fix expect_fun_call producing invalid suggestions

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