SourceManager intialization tweaks.

- Open files before calling stat on them.
- Go through FileManager for getting the buffer of named pipes. It has the
  necessary plumbing to deal with "volatile" files.
- Print the cause when stdin reading fails. The only case I can imagine where
  this happens is when stdin is wired to a device file, so no test case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188178 91177308-0d34-0410-b5e6-96231b3b80d8