[Sema] Make BreakContinueFinder handle nested loops.

We don't care about break or continue statements that aren't
associated with the current loop, so make sure the visitor
doesn't find them.

Fixes https://bugs.llvm.org/show_bug.cgi?id=32648 .

Differential Revision: https://reviews.llvm.org/D34568

llvm-svn: 307051