[libc++] Declare noop_coroutine() with _LIBCPP_INLINE_VISIBILITY

Summary:
It was defined with the right visibility, but declared without any visibility.
This function was left out of a prior revision that did the same to several
functions in <compare> (r336665) because the compiler I used didn't support
coroutines. This reinforces the need for automated checks -- there might
still be several cases of this throughout the library.

Reviewers: EricWF

Subscribers: modocache, christof, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D49145

llvm-svn: 336709