[clang][doxygen] Fix false -Wdocumentation warning for tag typedefs

For tag typedefs like this one:

/*!
@class Foo
*/
typedef class { } Foo;

clang -Wdocumentation gives:

warning: '@class' command should not be used in a comment attached to a
non-struct declaration [-Wdocumentation]

... while doxygen seems fine with it.

Differential Revision: https://reviews.llvm.org/D74746