SourceManager: Use setMainFileID() consistently

Eliminate createMainFileID() / createMainFileIDForMemBuffer() utility
functions. These didn't add much convenience and conflated two distinct
operations.

This change makes things easier to follow by providing a consistent interface
and getting rid of a bunch of cast-to-voids.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209266 91177308-0d34-0410-b5e6-96231b3b80d8