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

Summary:
The opening square of an inline asm clobber was being annotated as an ObjCExpr.
This caused, amongst other things, the ObjCGuesser to guess header files
containing that pattern as ObjC files.

Reviewers: benhamilton

Reviewed By: benhamilton

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D54111

llvm-svn: 346756