Fix doxygen warnings
Doxygen doesn't recognize <blockquote> html tags. Added support for <blockquote>
and Markdown was introduce with doxygen 1.8.0. This patch replaces blockquote
with \par for compatibility with previous versions.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@187067 91177308-0d34-0410-b5e6-96231b3b80d8