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