Try to fix warnings to escape '@' as '\@'. [-Wdocumentation-html]

  clang/include/clang/Format/Format.h:185:8: error: HTML tag 'tt' requires an end tag [-Werror,-Wdocumentation-html]
    /// <tt>@property (readonly)</tt> instead of <tt>@property(readonly)</tt>.
        ~^~~

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206893 91177308-0d34-0410-b5e6-96231b3b80d8