[ThinLTO] Add an auto-hide feature

When a symbol is not exported outside of the
DSO, it is can be hidden. Usually we try to internalize
as much as possible, but it is not always possible, for
instance a symbol can be referenced outside of the LTO
unit, or there can be cross-module reference in ThinLTO.

This is a recommit of r293912 after fixing build failures,
and a recommit of r293918 after fixing LLD tests.

Differential Revision: https://reviews.llvm.org/D28978

llvm-svn: 293970

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -971,13 +971,13 @@ static unsigned getEncodedLinkage(const GlobalValue &GV) {
 static uint64_t getEncodedGVSummaryFlags(GlobalValueSummary::GVFlags Flags) {
   uint64_t RawFlags = 0;
 
-  RawFlags |= Flags.NotEligibleToImport; // bool
-  RawFlags |= (Flags.LiveRoot << 1);
   // Linkage don't need to be remapped at that time for the summary. Any future
   // change to the getEncodedLinkage() function will need to be taken into
   // account here as well.
-  RawFlags = (RawFlags << 4) | Flags.Linkage; // 4 bits
-
+  RawFlags |= Flags.Linkage;                    // 4 bits linkage
+  RawFlags |= (Flags.NotEligibleToImport << 4); // bool
+  RawFlags |= (Flags.LiveRoot << 5);            // bool
+  RawFlags |= (Flags.AutoHide << 6);            // bool
   return RawFlags;
 }