[LTO] Drop non-prevailing definitions only if linkage is not local or appending

Description

[LTO] Drop non-prevailing definitions only if linkage is not local or appending

Summary:
This fixes PR 37422

In ELF, non-weak symbols can also be non-prevailing. In this particular
PR, the __llvm_profile_* symbols are non-prevailing but weren't getting
dropped - causing multiply-defined errors with lld.

Also add a test, strong_non_prevailing.ll, to ensure that multiple
copies of a strong symbol are dropped.

To fix the test regressions exposed by this fix,

  • do not mark prevailing copies for symbols with 'appending' linkage.

There's no one prevailing copy for such symbols.

  • fix the prevailing version in dead-strip-fulllto.ll
  • explicitly pass exported symbols to llvm-lto in fumcimport.ll and

funcimport_var.ll

Reviewers: tejohnson, pcc

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

Differential Revision: https://reviews.llvm.org/D54125

Details