LLVM

A lot of xml, the C++ changes are in 8d4650b, [1d188d0 through 854a817](https://github.com/eggrobin/Principia/compare/eggrobin:f178eee...854a817), and 318d205.
This is not stable yet:
* `testing_utilities` and `geometry` cause the compiler to crash somewhere deep in the gtest headers;
* the Google headers are dark and full of warnings;
* some tests for `quantities` fail because calls to `sprintf_s` get garbled somehow.

The `sprintf_s` garbling also makes the output from glog and gtest mostly useless (`3e-308` might actually mean `1e+00`), so we don't want to actually run this build at the moment.

However, besides the two compiler crashes, it new compiles the whole thing without errors, and without warnings in our code (all remaining warnings are in the Google headers).
At this point we can probably use it regularly to enforce better compliance with the standard and demystify compiler errors, so it is useful to keep the new build config.