[llvm-objdump] - Stop reporting bogus section IDs.

[grimar]

Summary

Imagine we have a file with few sections, and one of them is .foo
with index N != 0.

Problem is that when llvm-objdump is given a -section=.foo parameter
it lists .foo as a section at index 0. That makes impossible to write
test cases which needs to find the index of the particular section,
while ignoring dumping of others.

The patch fixes that.

[jhenderson]

LGTM, with a couple of minor improvements to the test.

test/tools/llvm-objdump/X86/section-index.s

9

I'd update the comment to explicitly mention "when requesting a specific section".

13

Maybe you might want to check that no other sections are printed?

test/tools/llvm-objdump/X86/section-index.s

+# RUN: llvm-mc %s -filetype=obj -triple=x86_64-pc-linux -o %t
+
+# RUN: llvm-objdump -section-headers %t | FileCheck %s
+# CHECK:     Idx Name
+# CHECK:      3  .foo
+# CHECK-NEXT: 4  .bar
+# CHECK-NEXT: 5  .zed
+
+## Check we report the valid section index for .bar
+# RUN: llvm-objdump -section-headers -section=.bar %t \
+# RUN:   | FileCheck %s --check-prefix=BAR
+# BAR:      Idx Name
+# BAR-NEXT:  4  .bar
+
+.section .foo, "ax", %progbits
+nop
+
+.section .bar, "ax", %progbits
+nop
+
+.section .zed, "ax", %progbits
+nop

tools/llvm-objdump/llvm-objdump.cpp

for (const RelocationRef &Reloc : Section.relocations()) {
              << valuestr << "\n";
     }
   }
 }
 
 void llvm::PrintSectionHeaders(const ObjectFile *Obj) {
   outs() << "Sections:\n"
             "Idx Name          Size      Address          Type\n";
-  unsigned i = 0;
   for (const SectionRef &Section : ToolSectionFilter(*Obj)) {
     StringRef Name;
     error(Section.getName(Name));
     uint64_t Address = Section.getAddress();
     uint64_t Size = Section.getSize();
     bool Text = Section.isText();
     bool Data = Section.isData();
     bool BSS = Section.isBSS();
     std::string Type = (std::string(Text ? "TEXT " : "") +
                         (Data ? "DATA " : "") + (BSS ? "BSS" : ""));
-    outs() << format("%3d %-13s %08" PRIx64 " %016" PRIx64 " %s\n", i,
-                     Name.str().c_str(), Size, Address, Type.c_str());
-    ++i;
+    outs() << format("%3d %-13s %08" PRIx64 " %016" PRIx64 " %s\n",
+                     Section.getIndex(), Name.str().c_str(), Size, Address,
+                     Type.c_str());
   }
 }
 
 void llvm::PrintSectionContents(const ObjectFile *Obj) {
   std::error_code EC;
   for (const SectionRef &Section : ToolSectionFilter(*Obj)) {
     StringRef Name;
     StringRef Contents;