Silence unintended fallthrough diagnostic on a case label preceded with a normal label.

Summary:
It's unlikely that a fallthrough is unintended in the following code:
switch (n) {
...
  label:
  case 1:
...
    goto label;
...
}

Reviewers: rsmith, doug.gregor

Reviewed By: doug.gregor

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D329

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173486 91177308-0d34-0410-b5e6-96231b3b80d8