[ThinLTO] Auto-hide prevailing linkonce_odr only when all copies eligible

Summary:
We hit undefined references building with ThinLTO when one source file
contained explicit instantiations of a template method (weak_odr) but
there were also implicit instantiations in another file (linkonce_odr),
and the latter was the prevailing copy. In this case the symbol was
marked hidden when the prevailing linkonce_odr copy was promoted to
weak_odr. It led to unsats when the resulting shared library was linked
with other code that contained a reference (expecting to be resolved due
to the explicit instantiation).

Add a CanAutoHide flag to the GV summary to allow the thin link to
identify when all copies are eligible for auto-hiding (because they were
all originally linkonce_odr global unnamed addr), and only do the
auto-hide in that case.

Most of the changes here are due to plumbing the new flag through the
bitcode and llvm assembly, and resulting test changes. I augmented the
existing auto-hide test to check for this situation.

Reviewers: pcc

Subscribers: mehdi_amini, inglorion, eraman, dexonsmith, arphaman, dang, llvm-commits, steven_wu, wmi

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59709

llvm-svn: 360466

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -893,8 +893,9 @@ static GlobalValueSummary::GVFlags getDecodedGVSummaryFlags(uint64_t RawFlags,
   // values as live.
   bool Live = (RawFlags & 0x2) || Version < 3;
   bool Local = (RawFlags & 0x4);
+  bool AutoHide = (RawFlags & 0x8);
 
-  return GlobalValueSummary::GVFlags(Linkage, NotEligibleToImport, Live, Local);
+  return GlobalValueSummary::GVFlags(Linkage, NotEligibleToImport, Live, Local, AutoHide);
 }
 
 // Decode the flags for GlobalVariable in the summary