Set IgnoreLLVMUsed to false in CallGraph::addToCallGraph()

[rampitec]

Summary

clang++ uses llvm.compiler.used in certain cases to preserve
symbol which is fully inlined. D96087 has resulted in undefined
symbols in such cases. Set it to false by default to preserve
old behavior but keep the option for specific uses where we
want to ignore these (e.g. to detect a potential indirect call
to a function).

[SeanP]

LGTM

llvm/lib/Analysis/CallGraph.cpp

 void CallGraph::addToCallGraph(Function *F) {
   CallGraphNode *Node = getOrInsertFunction(F);
 
   // If this function has external linkage or has its address taken and
   // it is not a callback, then anything could call it.
   if (!F->hasLocalLinkage() ||
       F->hasAddressTaken(nullptr, /*IgnoreCallbackUses=*/true,
                          /* IgnoreAssumeLikeCalls */ true,
-                         /* IgnoreLLVMUsed */ true))
+                         /* IgnoreLLVMUsed */ false))
     ExternalCallingNode->addCalledFunction(nullptr, Node);
 
   populateCallGraphNode(Node);
 }
 
 void CallGraph::populateCallGraphNode(CallGraphNode *Node) {
   Function *F = Node->getFunction();
 

llvm/test/Analysis/CallGraph/llvm-used.ll

 ; RUN: opt < %s -print-callgraph -disable-output 2>&1 | FileCheck %s
+
+; The test will report used1 and used2 functions as used on the grounds
+; of llvm.*.used references. Passing IgnoreLLVMUsed = true into the
+; Function::hasAddressTaken() in the CallGraph::addToCallGraph() has to
+; change their uses to zero.
+
 ; CHECK: Call graph node <<null function>><<{{.*}}>>  #uses=0
+; CHECK-NEXT:  CS<None> calls function 'used1'
+; CHECK-NEXT:  CS<None> calls function 'used2'
 ; CHECK-NEXT:  CS<None> calls function 'unused'
 ; CHECK-EMPTY:
 ; CHECK-NEXT:   Call graph node for function: 'unused'<<{{.*}}>>  #uses=1
 ; CHECK-EMPTY:
-; CHECK-NEXT:   Call graph node for function: 'used1'<<{{.*}}>>  #uses=0
+; CHECK-NEXT:   Call graph node for function: 'used1'<<{{.*}}>>  #uses=1
 ; CHECK-EMPTY:
-; CHECK-NEXT:   Call graph node for function: 'used2'<<{{.*}}>>  #uses=0
+; CHECK-NEXT:   Call graph node for function: 'used2'<<{{.*}}>>  #uses=1
 ; CHECK-EMPTY:
 
 @llvm.used = appending global [1 x i8*] [i8* bitcast (void ()* @used1 to i8*)]
 @llvm.compiler.used = appending global [1 x void()*] [void ()* @used2]
 @array = appending global [1 x i8*] [i8* bitcast (void ()* @unused to i8*)]
 
 define internal void @used1() {
 entry: