Switch to using BalancedDelimiterTracker to get better diagnostics for
unbalanced delimiters.  This still does not handle any of the attributes, simply
improves the parsing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239758 91177308-0d34-0410-b5e6-96231b3b80d8