Modernize doc comments for SourceMgr.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211086 91177308-0d34-0410-b5e6-96231b3b80d8
Add an overload for SourceMgr::PrintMessage that takes an existing diagnostic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211087 91177308-0d34-0410-b5e6-96231b3b80d8