Frontend: Add support for reading named pipes as the main file.

 - The whole {File,Source}Manager is built around wanting to pre-determine the
   size of files, so we can't fit this in naturally. Instead, we handle it like
   we do STDIN, where we just replace the main file contents upfront.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@167419 91177308-0d34-0410-b5e6-96231b3b80d8