swift234

Use 'unsigned' to represent a present input buffer ID.

LLVM's SourceMgr uses 'int' for buffer IDs, but -1 is a special placeholder
value that means "no such buffer". Swift's SourceManager then makes a
distinction between 'unsigned' and 'int' -- an unsigned buffer ID always
refers to an actual buffer, while an int ID may be -1. Respect this in
SourceFile, which uses Optional to represent a "potential" buffer.

Swift SVN r9239