Correct Doxygen syntax for inline code

Correct Doxygen syntax for inline code

There is no syntax like {@code ...} in Doxygen, @code is a block command
that ends with @endcode, and generally these are not enclosed in braces.
The correct syntax for inline code snippets is @c <code>.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D98665