Add FILE_SHARE_WRITE to openFileForRead.

This should fix the windows bots. It looks like the failing tests are of the
form

prog1 > file
prog2 file

and prog2 fails trying to read the file. The best fix would probably be to close
stdout/stderr in prog1, but it was not the intention of 186511 to change this,
so just restore the old behavior for now.

llvm-svn: 186530