[ThinLTO] Summarize vcall_visibility metadata

Summary:
Second patch in series to support Safe Whole Program Devirtualization
Enablement, see RFC here:
http://lists.llvm.org/pipermail/llvm-dev/2019-December/137543.html

Summarize vcall_visibility metadata in ThinLTO global variable summary.

Depends on D71907.

Reviewers: pcc, evgeny777, steven_wu

Subscribers: mehdi_amini, Prazek, inglorion, hiraditya, dexonsmith, arphaman, ostannard, llvm-commits, cfe-commits, davidxl

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D71911