[AST] Remove StmtRange in favor of an iterator_range.

[AST] Remove StmtRange in favor of an iterator_range.

StmtRange was just a convenient wrapper for two StmtIterators before
we had real range support. This removes some of the implicit conversions
StmtRange had leading to slightly more verbose code but also should make
more obvious what's going on. No functional change intended.

llvm-svn: 242615