[Analysis] -Wunreachable-code shouldn't fire on the increment of a foreach loop

Summary:
The idea is that the code here isn't written, so doesn't indicate a bug.
Similar to code expanded from macros.

This means the warning no longer fires on this code:
  for (auto C : collection) {
    process(C);
    return;
  }
  handleEmptyCollection();
Unclear whether this is more often a bug or not in practice, I think it's a
reasonable idiom in some cases.
Either way, if we want to warn on "loop that doesn't loop", I think it should be
a separate warning, and catch `while(1) break;`

Reviewers: ilya-biryukov, ioeric

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58134

llvm-svn: 354102