Revert "[Support] Add RetryAfterSignal helper function" and subsequent fix

The fix in r306003 uncovered a pretty fundamental problem that libc++
implementation of std::result_of does not handle the prototype of
open(2) correctly (presumably because it contains ...). This makes the
whole function unusable in its current form, so I am also reverting the
original commit (r305892), which introduced the function, at least until
I figure out a way to solve the libc++ issue.

llvm-svn: 306005