logging: support building with gcc-4.9
logging: support building with gcc-4.9

Summary:
Versions of gcc prior to 5.x only support C++11 constexpr functions, and
require that they consist of only a single return statement.

This updates isLogLevelFatal() to meet those requirements so that it can still
be compiled with gcc 4.9.

Reviewed By: ikobzar

Differential Revision: D5350304

fbshipit-source-id: 2a9c256236c484f8c3d5f83690fde8b7333aa4fc

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