IR: Eliminate non-determinism in the module summary analysis.

Also make the summary ref and call graph vectors immutable. This means
a smaller API surface and fewer places to audit for non-determinism.

Differential Revision: https://reviews.llvm.org/D27875

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290200 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed