Frontend: Sink named pipe logic from CompilerInstance down to FileManager

Remove compilicated logic from CompilerInstance::InitializeSourceManager
to deal with named pipes, updating FileManager::getBufferForFile to
handle it in a more straightforward way. The existing test at
clang/test/Misc/dev-fd-fs.c covers the new behaviour (just like it did
the old behaviour).

Differential Revision: https://reviews.llvm.org/D90733