logging: fix compiler compatibility for one more constexpr function
logging: fix compiler compatibility for one more constexpr function

Summary:
Update LogLevel's operator+() to consist only of a single return statement.
This is required for pre-C++14 compiler support.

Reviewed By: yfeldblum

Differential Revision: D5368256

fbshipit-source-id: 9ecbcde5edd1d0b3e7580d6263ad926e44908219

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