clang-format: [JS] support optional methods.

Optional methods use ? tokens like this:

  interface X { y?(): z; }

It seems easiest to detect and disambiguate these from ternary
expressions by checking if the code is in a declaration context. Turns
out that that didn't quite work properly for interfaces in Java and JS,
and for JS file root contexts.

Patch by Martin Probst, thank you.

llvm-svn: 236488