[ThinLTO] Add a recursive step in Metadata lazy-loading

Summary:
Without this, we're stressing the RAUW of unique nodes,
which is a costly operation. This is intended to limit
the number of RAUW, and is very effective on the total
link-time of opt with ThinLTO, before:

  real 4m4.587s  user 15m3.401s  sys 0m23.616s

after:

  real 3m25.261s user 12m22.132s sys 0m24.152s

Reviewers: tejohnson, pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28751

llvm-svn: 292420