Add the location of Decls to ast dump.

While investigating some debug info issues, Eric and I came across a
particular template case where the location of a decl was quite
different from the range of the same decl. It might've been rather
helpful if the dumper had actually showed us this.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@205396 91177308-0d34-0410-b5e6-96231b3b80d8