Add introductory paragraph to the C++ status page

Use internal links to provide easier access to recent and ongoing work.

Also shift up the order of standards in the page title in order to avoid web
search results focusing on C++98 in the summary.

This is done to highlight the modern standards support in clang that was
previously languishing at the bottom of the page.

  "C++98/03 is sooooo yesterday" - dgregor

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196565 91177308-0d34-0410-b5e6-96231b3b80d8