[clang-format] Don't detect call to ObjC class method as C++11 attribute specifier

Summary:
Previously, clang-format detected something like the following as a C++11 attribute specifier.

  @[[NSArray class]]

instead of an array with an Objective-C method call inside. In general, when the attribute specifier checking runs, if it sees 2 identifiers in a row, it decides that the square brackets represent an Objective-C method call. However, here, `class` is tokenized as a keyword instead of an identifier, so this check fails.

To fix this, the attribute specifier first checks whether the first square bracket has an "@" before it. If it does, then that square bracket is not the start of a attribute specifier because it is an Objective-C array literal. (The assumption is that @[[.*]] is not valid C/C++.)

Contributed by rkgibson2.

Reviewers: benhamilton

Reviewed By: benhamilton

Subscribers: aaron.ballman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64632

llvm-svn: 366267