clang-format: Detect braced lists in subscript expressions.

Before (even with Style.Cpp11BracedListStyle):
  f(MyMap[{ composite, key }]);

After:
  f(MyMap[{composite, key}]);

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190678 91177308-0d34-0410-b5e6-96231b3b80d8