Extend the -Warray-bounds workaround in FixedString.h to gcc-5.
Extend the -Warray-bounds workaround in FixedString.h to gcc-5.

Summary: FixedString.h has a workaround for a gcc bug regarding erroneous -Werror=array-bounds errors. This commit extends the workaround to the gcc-5 series, which sadly is still broken. Life gets better with gcc-6.

Reviewed By: luciang

Differential Revision: D5210633

fbshipit-source-id: 4fcf080d18ddc573636a3e58b61a97167a8c06ce

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