logging: fix issues detecting XLOG(FATAL) statements as noreturn

logging: fix issues detecting XLOG(FATAL) statements as noreturn

Summary:
Update the FB_LOG() and XLOG() macros so that FATAL log messages are correctly
detected as not returning, by both clang and gcc.

We have to ensure that both sides of the log statement check (log message
enabled or disabled) evaluate to `[[noreturn]]` expressions.  I did try
updating the log check itself so that it could be constexpr detected as always
passing, but this was not sufficient.

Reviewed By: wez

Differential Revision: D5290780

fbshipit-source-id: 773a56a8392dfd7c310d5d84fc9311e66edf99cb