[clang-format] Do not treat asm clobber [ as ObjCExpr, refined

Summary:
r346756 refined clang-format to not treat the `[` in `asm (...: [] ..)` as an
ObjCExpr. However that's not enough, as we might have a comma-separated list of
such clobbers as in the newly added test.
This updates the detection to instead look at the Line's first token being `asm`
and not mark `[`-s as ObjCExprs in this case.

Reviewers: djasper, benhamilton

Reviewed By: djasper, benhamilton

Subscribers: benhamilton, cfe-commits

Differential Revision: https://reviews.llvm.org/D54795

llvm-svn: 347465