C++1y: support for 'switch' statements in constexpr functions. This is somewhat
inefficient; we perform a linear scan of switch labels to find the one matching
the condition, and then walk the body looking for that label. Both parts should
be straightforward to optimize.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181671 91177308-0d34-0410-b5e6-96231b3b80d8