Fix implementation of ::abs and std::abs LWG 2192.

Summary:
All overloads of `::abs` and `std::abs` must be present in both `<cmath>` and `<cstdlib>`. This is problematic to implement because C defines `fabs` in `math.h` and `labs` in `stdlib.h`. This introduces a circular dependency between the two headers. 

This patch implements that requirement by moving `abs` into `math.h` and making `stdlib.h` include `math.h`. In order to get the underlying C declarations from the "real" `stdlib.h` inside our `math.h` we need some trickery. Specifically we need to make `stdlib.h` include next itself.

Suggestions for a cleaner implementation are welcome.

Reviewers: mclow.lists, ldionne

Reviewed By: ldionne

Subscribers: krytarowski, fedor.sergeev, dexonsmith, jdoerfert, jsji, libcxx-commits

Differential Revision: https://reviews.llvm.org/D60097

llvm-svn: 359020

0 files changed, 0 insertions, 0 deletions