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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306005 91177308-0d34-0410-b5e6-96231b3b80d8