[Sema][Comments] Add support for TypeAliasTemplate

Emit proper diagnostics when -Wdocumentation is used with constructs such as:

  template<typename T>
  using fn = int(T aaa, int ccc);

Previously clang wouldn't recognize the function and complain with
'comment that is not attached to a function declaration'.

Differential Revision: https://reviews.llvm.org/D23860

rdar://problem/27300695

llvm-svn: 279754