[Support] Fix return type deduction in RetryAfterSignal

The default value of the ResultT template argument (which was there only
to avoid spelling out the long std::result_of template multiple times)
was being overriden by function call template argument deduction. This
manifested itself as a compiler error when calling the function as
FILE *X = RetryAfterSignal(nullptr, fopen, ...)
because the function would try to assign the result of fopen to
nullptr_t, but a more insidious side effect was that
RetryAfterSignal(-1, read, ...) would return "int" instead of "ssize_t",
losing precision along the way.

I fix this by having the function take the argument in a way that
prevents argument deduction from kicking in and add a test that makes
sure the return type is correct.

llvm-svn: 306003