C++1y constexpr extensions, round 1: Allow most forms of declaration and

C++1y constexpr extensions, round 1: Allow most forms of declaration and
statement in constexpr functions. Everything which doesn't require variable
mutation is also allowed as an extension in C++11. 'void' becomes a literal
type to support constexpr functions which return 'void'.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180022 91177308-0d34-0410-b5e6-96231b3b80d8