[Doc parsing]: This patch adds <Declaration> tag to

[Doc parsing]: This patch adds <Declaration> tag to
XML comment for declarations which pretty-prints
declaration. I had to XFAIL one test annotate-comments.cpp.
This test is currently unmaintainable as written.
Dmitri G., can you see what we can do about this test.
We should change this test such that adding a new tag does not wreck
havoc to the test.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@166130 91177308-0d34-0410-b5e6-96231b3b80d8