Rearrange condition handling so that semantic checks on a condition variable

are performed before the other substatements of the construct are parsed,
rather than deferring them until the end. This allows better error recovery
from semantic errors in the condition, improves diagnostic order, and is a
prerequisite for C++17 constexpr if.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@273548 91177308-0d34-0410-b5e6-96231b3b80d8