[Sema][Comments] Support @param with c++ 'using' keyword

Give appropriate warnings with -Wdocumentation for @param comments
that refer to function aliases defined with 'using'. Very similar
to typedef's behavior. This does not add support for
TypeAliasTemplateDecl yet.

Differential Revision: https://reviews.llvm.org/D23783

rdar://problem/27300695

llvm-svn: 279662