clang-format: Handle "if constexpr".

c++1z adds the following constructions to the language:

  if constexpr (cond)
    statement1;
  else if constexpr (cond)
    statement2;
  else if constexpr (cond)
    statement3;
  else
    statement4;

A first version of this was proposed in reviews.llvm.org/D26953 by
Francis Visoiu Mistrih, but never commited. This patch additionally
fixes the behavior when allowing short if statements on a single line
and was authored by Jacob Bandes-Storch. Thank you to both authors.

llvm-svn: 305666