AIX provides additional definitions in the system libc float.h that we
would like to be available to users, so we need to include_next through,
similar to what is done on some other platforms.
We also adjust the guards for some definitions which are restricted
based on language level to also be provide with the _ALL_SOURCE feature
test macro on AIX, similar to what is done by the platform float.h
header, so we don't run into cases where we don't provide the compiler
macro but still have a different definition from the system.
Differential Revision: https://reviews.llvm.org/D117935
David Tenty