This is the only way not to confuse the diagnostics engine of upgraded clang.

Also, this gives us a few advantages:
  * We can compare more precisely the source locations of diagnostics;
  * We can merge the code completion code path which works with file entries;
  * We can rely better when specifying //expected-note-s in different files.

(cherry picked from commit b91217ed)

(cherry picked from commit 5a816dc8)

(cherry picked from commit 11030ba6)

I cannot handle unions. (Which is probably why we will have variant.)

Travis: Re-enable build on Xcode - 6.4. This allows testing on 10.9.5 as well as older versions of libc++.

std::array, using aggregate init, allows *less* initializers with remaining ones being {} initialized.
That is evil for coordinates, where we want {12.,13.,14.} for 3-dim coords, not 4-dim coord!
Use the std::tuple approach of only making the ctor available if the right number of args is passed.
Else, static_assert to be helpful.