clang-format: [JS] improve calculateBraceType heuristic

Summary:

calculateBraceTypes decides for braced init for empty brace pairs ({}).
In context of a function declaration, this incorrectly classifies empty
function or method bodies as braced inits, leading to missing wraps:

    class C {
      foo() {}[bar]() {}
    }

Where code should have wrapped after "}", before "[". This change adds
another piece of contextual information in that braces following closing
parentheses must always be the opening braces of function blocks. This
fixes brace detection for methods immediately followed by brackets
(computed property declarations), but also curlies.

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D33714

llvm-svn: 304290