Cloning: Reduce complexity of debug info cloning and fix correctness issue.

Commit r260791 contained an error in that it would introduce a cross-module
reference in the old module. It also introduced O(N^2) complexity in the
module cloner by requiring the entire module to be visited for each function.
Fix both of these problems by avoiding use of the CloneDebugInfoMetadata
function (which is only designed to do intra-module cloning) and cloning
function-attached metadata in the same way that we clone all other metadata.

Differential Revision: http://reviews.llvm.org/D18583

llvm-svn: 264935