lto.h: Use lto_bool_t instead of int to restore the ABI

This reverts commit r193255 and instead creates an lto_bool_t typedef
that points to bool, _Bool, or unsigned char depending on what is
available.  Only recent versions of MSVC provide a stdbool.h header.

Reviewers: rafael.espindola

Differential Revision: http://llvm-reviews.chandlerc.com/D2019