[llvm-objdump][CallGraphInfo] Get .callgraph section

[necipfazil]

Summary

Get the .callgraph section. Warn if there is no .callgraph section in the
binary.

Upcoming patches will parse and print information from this section.

Patch 4/6: Extract call graph information from binary

Original RFC: https://lists.llvm.org/pipermail/llvm-dev/2021-June/151044.html
Updated RFC: https://lists.llvm.org/pipermail/llvm-dev/2021-July/151739.html

[morehouse]

LGTM

llvm/test/tools/llvm-objdump/ELF/call-graph-info-warn-no-callgraph-section.test

+## Tests that --call-graph-info warns if there is no .callgraph section.
+
+# RUN: llvm-mc %s -filetype=obj -triple=x86_64-pc-linux -o %t
+# RUN: llvm-objdump --call-graph-info %t 2>&1 | FileCheck %s -DFILE=%t
+
+# CHECK: [[FILE]]: file format elf64-x86-64
+# CHECK-NEXT: llvm-objdump: warning: '[[FILE]]': there is no .callgraph section
+# CHECK-NOT: INDIRECT TARGET TYPES (TYPEID [FUNC_ADDR,])
+# CHECK-NOT: INDIRECT CALL TYPES (TYPEID [CALL_SITE_ADDR,])
+
+.text
+.globl _Z3foov
+.type _Z3foov,@function
+_Z3foov:
+ callq _Z3foov@PLT

llvm/tools/llvm-objdump/llvm-objdump.cpp

void objdump::printSymbol(const ObjectFile *O, const SymbolRef &Symbol,
   else
     outs() << ' ' << Name << '\n';
 }
 
 static void printCallGraphInfo(const ObjectFile *Obj) {
   // Get function info through disassembly.
   disassembleObject(Obj, /*InlineRelocs=*/false);
 
+  // Get the .callgraph section.
+  StringRef CallGraphSectionName(".callgraph");
+  Optional<object::SectionRef> CallGraphSection;
+  for (auto Sec : ToolSectionFilter(*Obj)) {
+    StringRef Name;
+    if (Expected<StringRef> NameOrErr = Sec.getName())
+      Name = *NameOrErr;
+    else
+      consumeError(NameOrErr.takeError());
+
+    if (Name == CallGraphSectionName) {
+      CallGraphSection = Sec;
+      break;
+    }
+  }
+  if (!CallGraphSection)
+    reportWarning("there is no .callgraph section", Obj->getFileName());
+
   // Print function entry to indirect call site addresses mapping from disasm.
   outs() << "\n\nINDIRECT CALL SITES (CALLER_ADDR [CALL_SITE_ADDR,])";
   for (const auto &El : FuncInfo) {
     auto CallerPc = El.first;
     auto FuncIndirCallSites = El.second.IndirectCallSites;
     if (!FuncIndirCallSites.empty()) {
       outs() << "\n" << format("%lx", CallerPc);
       for (auto IndirCallSitePc : FuncIndirCallSites)