Don't trust st_size of a character device. This fixes using
/dev/stdin as an input when stdin is connected to a tty, for example.

No test, because it's difficult to write a reasonably portable test
for this. /dev/stdin isn't a character device when stdin is redirected
from a file or connected to a pipe.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175542 91177308-0d34-0410-b5e6-96231b3b80d8