llvm-mt: Merge manifest namespaces.

mt.exe performs a tree merge where certain element nodes are combined
into one.  This introduces the possibility of xml namespaces conflicting
with each other.  The original mt.exe has a hierarchy whereby certain
namespace names can override others, and nodes that would then end up in
ambigious namespaces have their namespaces explicitly defined.  This
namespace handles this merging process.

llvm-svn: 311215