clang-format: [JS] simplify import/export.

Summary:
Change `import` and `export` parsing to special case the renaming
syntax (`import x, {y as bar} ...`, `export {x}`) and otherwise just
parse a regular structural element.

This simplifies the code a bit and should be more correct - it's easier
to recognise the specific import syntax than to recognise arbitrary
expressions and declarations.

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: http://reviews.llvm.org/D19242

llvm-svn: 266743