[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

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -1028,8 +1028,8 @@ static uint64_t getEncodedGVSummaryFlags(GlobalValueSummary::GVFlags Flags) {
 }
 
 static uint64_t getEncodedGVarFlags(GlobalVarSummary::GVarFlags Flags) {
-  uint64_t RawFlags =
-      Flags.MaybeReadOnly | (Flags.MaybeWriteOnly << 1) | (Flags.Constant << 2);
+  uint64_t RawFlags = Flags.MaybeReadOnly | (Flags.MaybeWriteOnly << 1) |
+                      (Flags.Constant << 2) | Flags.VCallVisibility << 3;
   return RawFlags;
 }