Fix a performance bug in the Linker.

Now that we hava a convinient place to keep it, remeber the set of
identified structs as we merge modules.

This speeds up the linking of all the bitcode files in clang with the
gold plugin and -plugin-opt=emit-llvm (i.e., link only, no codegen) from
5:25 minutes to 13.6 seconds!

Patch by Xiaofei Wan!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181104 91177308-0d34-0410-b5e6-96231b3b80d8
Optimize llvm-link too.

This takes the linking of almost all modules in a clang build from 6:32
to 0:19.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181105 91177308-0d34-0410-b5e6-96231b3b80d8