Implement clang support for indirect branch and address of label

Implement clang support for indirect branch and address of label
using the new LLVM support for this.  This is temporarily hiding
behind horrible and ugly #ifdefs until the time when the optimizer
is stable (hopefully a week or so).  Until then, lets make it "opt in" :)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@85446 91177308-0d34-0410-b5e6-96231b3b80d8