[ELF] Don't emit SHF_GNU_RETAIN on Solaris

The introduction of `SHF_GNU_RETAIN` has caused massive problems on Solaris.

Initially, as reported in Bug 49437, it caused dozens of testsuite failures
on both sparc and x86.  The objects were marked as `ELFOSABI_NONE`, but
`SHF_GNU_RETAIN` is a GNU extension. In the native Solaris ABI, that flag
(in the range for OS-specific values) is `SHF_SUNW_ABSENT` with a
completely different semantics, which confuses Solaris `ld` very much.

Later, the objects became (correctly) marked `ELFOSABI_GNU`, which Solaris
`ld` doesn't support, causing it to SEGV and break the build.  The linker
is currently being hardened to not accept non-native OS ABIs to avoid this.

The need for linker support is already documented in
`clang/include/clang/Basic/AttrDocs.td`, but not currently checked.

This patch avoids all this by not emitting `SHF_GNU_RETAIN` on Solaris at all.

Tested on `amd64-pc-solaris2.11`, `sparcv9-sun-solaris2.11`, and
`x86_64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D107747

(cherry picked from commit 7bbbf29)

Author: rorth

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -677,8 +677,9 @@ calcUniqueIDUpdateFlagsAndSize(const GlobalObject *GO, StringRef SectionName,
   }
 
   if (Retain) {
-    if (Ctx.getAsmInfo()->useIntegratedAssembler() ||
-        Ctx.getAsmInfo()->binutilsIsAtLeast(2, 36))
+    if ((Ctx.getAsmInfo()->useIntegratedAssembler() ||
+         Ctx.getAsmInfo()->binutilsIsAtLeast(2, 36)) &&
+        !TM.getTargetTriple().isOSSolaris())
       Flags |= ELF::SHF_GNU_RETAIN;
     return NextUniqueID++;
   }
@@ -855,8 +856,10 @@ static MCSection *selectELFSectionForGlobal(
     EmitUniqueSection = true;
     Flags |= ELF::SHF_LINK_ORDER;
   }
-  if (Retain && (Ctx.getAsmInfo()->useIntegratedAssembler() ||
-                 Ctx.getAsmInfo()->binutilsIsAtLeast(2, 36))) {
+  if (Retain &&
+      (Ctx.getAsmInfo()->useIntegratedAssembler() ||
+       Ctx.getAsmInfo()->binutilsIsAtLeast(2, 36)) &&
+      !TM.getTargetTriple().isOSSolaris()) {
     EmitUniqueSection = true;
     Flags |= ELF::SHF_GNU_RETAIN;
   }