Mark libc math shims with always_inline attribute (#6956)
* Mark libc math shims with always_inline attribute
We want these to always be inlined, so mark them with ... always_inline.
The compiler happened to do the right thing with them previously, but we
shouldn't depend on that.
<rdar://problem/30043258> master-next: IRGen/builtin_math.swift failing because sqrt(4) is not constant folded
