[patch] Add noexcept to Filesystem TS operators

[Jonathan Wakely]

This isn't terribly important, but these operators might as well be
noexcept, and committing it now isn't going to cause any problems
backporting anything to gcc-6-branch at the last minute.

I have a couple more patches for Filesystem TS issues coming too.

Tested x86_64-linux, committed to trunk.

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