"FileID" a concept that is now enforced by the compiler's type checker
instead of yet-another-random-unsigned floating around.
This is an important distinction from the "FileID" currently tracked by
SourceLocation. *That* FileID may refer to the start of a file or to a
chunk within it. The new FileID *only* refers to the file (and its
#include stack and eventually #line data), it cannot refer to a chunk.
FileID is a completely opaque datatype to all clients, only SourceManager
is allowed to poke and prod it.
llvm-svn: 62407