MFC r354460:

Merge commit e8316372b from llvm git (by Louis Dionne):

[libc++] Add `__truncating_cast` for safely casting float types to
integers

This is needed anytime we need to clamp an arbitrary floating point
value to an integer type.

Thanks to Eric Fiselier for the patch.

Differential Revision: https://reviews.llvm.org/D66836

llvm-svn: 370891

Merge commit b92deded8 from llvm git (by Louis Dionne):

[libc++] Move __clamp_to_integral to <cmath>, and harden against
min()/max() macros

llvm-svn: 370900

Merge commit 0ec6a4882 from llvm git (by Louis Dionne):

[libc++] Fix potential OOB in poisson_distribution

See details in the original Chromium bug report:
    https://bugs.chromium.org/p/chromium/issues/detail?id=994957

Together, these fix a security issue in libc++'s implementation of
std::poisson_distribution, which can be exploited to read data which is
out of bounds.

Note there are no programs in the FreeBSD base system that use
std::poisson_distribution, so this is only a possible issue for ports
and external programs which have been built against libc++. Therefore,
I am bumping __FreeBSD_version for the benefit of our port maintainers.

Requested by: emaste
Security: potential OOB read