Don't fixit/recover from -Wstatic-float-init when it's not an error.

Fix to change r173414 that lead to Clang changing const to constexpr even under
-Wno-static-float-init.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173835 91177308-0d34-0410-b5e6-96231b3b80d8