clang-format: [JS] Let fat arrows have 'Assignment' precedence.

clang-format: [JS] Let fat arrows have 'Assignment' precedence.

This is a more correct representation than using "Equality" introduced
in r238942 which was a quick fix to solve an actual regression.

According to the typescript spec, arrows behave like "low-precedence"
assignments.

Before:
  var a = a.aaaaaaa((a: a) => aaaaaaaaaaaaaaaaaaaaa(bbbbbbbbb) &&
                    aaaaaaaaaaaaaaaaaaaaa(bbbbbbb));
After:
  var a = a.aaaaaaa((a: a) => aaaaaaaaaaaaaaaaaaaaa(bbbbbbbbb) &&
                              aaaaaaaaaaaaaaaaaaaaa(bbbbbbb));

llvm-svn: 239137