cmath: Skip Libc for integral types in isinf, etc.

For std::isinf, the standard requires effectively calling isinf as
double from Libc for integral types. But integral types are never
infinite; we don't need to call Libc to return false.

Also short-circuit other functions where Libc won't have interesting
answers: signbit, fpclassify, isfinite, isnan, and isnormal.

I added correctness tests for integral types since we're no longer
deferring to Libc.

In review it was pointed out that in future revisions of the C++
standard we may add more types to std::is_arithmetic (e.g.,
std::is_fixed_point).  I'll leave it to a future commit to hack this to
allow using math functions on those.  We'll need to change things like
__libcpp_fpclassify anyway, so I'm not sure anything here would really
be future-proof.

https://reviews.llvm.org/D31561
rdar://problem/31361223

llvm-svn: 301060