Revert "Add a verifier check that rejects non-distinct DISubprogram function"

This reverts commit e17f52d.

while investigating bot breakage.

(cherry picked from commit 8055ae3)

Author: adrian-prantl

llvm/lib/IR/Verifier.cpp

@@ -2404,10 +2404,6 @@ void Verifier::visitFunction(const Function &F) {
                  "function must have a single !dbg attachment", &F, I.second);
         AssertDI(isa<DISubprogram>(I.second),
                  "function !dbg attachment must be a subprogram", &F, I.second);
-        AssertDI(cast<DISubprogram>(I.second)->isDistinct(),
-                 "function definition may only have a distinct !dbg attachment",
-                 &F);
-
         auto *SP = cast<DISubprogram>(I.second);
         const Function *&AttachedTo = DISubprogramAttachments[SP];
         AssertDI(!AttachedTo || AttachedTo == &F,

llvm/test/DebugInfo/Generic/2009-11-03-InsertExtractValue.ll

@@ -3,7 +3,7 @@
 !llvm.module.flags = !{!6}
 !llvm.dbg.cu = !{!5}
 
-!0 = distinct !DISubprogram(name: "bar", linkageName: "_ZN3foo3barEv", line: 3, isLocal: false, isDefinition: false, virtualIndex: 6, flags: DIFlagProtected | DIFlagPrototyped, isOptimized: false, scopeLine: 3, file: !4, scope: !1, type: !2)
+!0 = !DISubprogram(name: "bar", linkageName: "_ZN3foo3barEv", line: 3, isLocal: false, isDefinition: false, virtualIndex: 6, flags: DIFlagProtected | DIFlagPrototyped, isOptimized: false, scopeLine: 3, file: !4, scope: !1, type: !2)
 !1 = !DIFile(filename: "/foo", directory: "bar.cpp")
 !2 = !DISubroutineType(types: !3)
 !3 = !{null}