HeaderDoc: Support more of HeaderDoc documentation 
commands; top level tags such as @interface and
their 2nd level tags such as @coclass, etc.
// rdar://12379114


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176667 91177308-0d34-0410-b5e6-96231b3b80d8