Correctly check if a cursor is a declaration before returning its location/range...

Correctly check if a cursor is a declaration before returning its location/range in clang_getCursorLocation()/clang_getCursorExtent().  This fixes a horrible bug reported in
<rdar://problem/7961995> and <rdar://problem/7967123> where declarations with attributes
would get grossly annotated with the wrong tokens because the attribute would be interpreted
as if it was a Decl*.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@103581 91177308-0d34-0410-b5e6-96231b3b80d8