[patch] Implement std::experimental::source_location (sort of)

[Jonathan Wakely <jwakely at redhat dot com>]

This is a non-conforming implementation of
https://rawgit.com/cplusplus/fundamentals-ts/v2/fundamentals-ts.html#reflection.src_loc

It doesn't provide any column numbers, and fails to meet the
requirement that using current() in a NSDMI will refer to the location
of the constructor ... but maybe it's good enough until we get the
necessary front-end support. It is experimental, after all.

Thoughts?

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