[ThinLTO] Add Visibility bits to GlobalValueSummary::GVFlags

Imported functions and variable get the visibility from the module supplying the
definition.  However, non-imported definitions do not get the visibility from
(ELF) the most constraining visibility among all modules (Mach-O) the visibility
of the prevailing definition.

This patch

* adds visibility bits to GlobalValueSummary::GVFlags
* computes the result visibility and propagates it to all definitions

Protected/hidden can imply dso_local which can enable some optimizations (this
is stronger than GVFlags::DSOLocal because the implied dso_local can be
leveraged for ELF -shared while default visibility dso_local has to be cleared
for ELF -shared).

Note: we don't have summaries for declarations, so for ELF if a declaration has
the most constraining visibility, the result visibility may not be that one.

Differential Revision: https://reviews.llvm.org/D92900

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -1044,7 +1044,8 @@ static uint64_t getEncodedFFlags(FunctionSummary::FFlags Flags) {
   return RawFlags;
 }
 
-// Decode the flags for GlobalValue in the summary
+// Decode the flags for GlobalValue in the summary. See getDecodedGVSummaryFlags
+// in BitcodeReader.cpp.
 static uint64_t getEncodedGVSummaryFlags(GlobalValueSummary::GVFlags Flags) {
   uint64_t RawFlags = 0;
 
@@ -1058,6 +1059,8 @@ static uint64_t getEncodedGVSummaryFlags(GlobalValueSummary::GVFlags Flags) {
   // account here as well.
   RawFlags = (RawFlags << 4) | Flags.Linkage; // 4 bits
 
+  RawFlags |= (Flags.Visibility << 8); // 2 bits
+
   return RawFlags;
 }