TypeSourceInfo is a thin wrapper around TypeLocs. Notionally, the best
place for it to live would be TypeLoc.h, but Decl.h requires it to be
complete, so it needs to be lower in the dependency graph. Type.h seems
like the next best place.
By itself, this change has no impact on build time, because it doesn't
remove a single dependency edge from a .cpp file to a .h file, but it is
an incremental step towards making the AST headers less interdependent.