Fix llvm/lib/ObjCopy, llvm/llvm-ifs: c++20 compatibility

[angelomatnigoogle]

Summary

Cleanup: avoid referring to std::vector<T> members when T is incomplete.

This is not legal
according to the C++ standard, and causes build errors in particular in C++20
mode. Fix it by defining the vector's type before using the vector.

[angelomatnigoogle]

Clean up commit message body

[saugustine]

This is a straight forward change and I don't believe the libfuzzer failures are related.

Accepting.

llvm/lib/ObjCopy/MachO/MachOObject.h

struct Section {
   uint32_t NReloc = 0;
   uint32_t Flags = 0;
   uint32_t Reserved1 = 0;
   uint32_t Reserved2 = 0;
   uint32_t Reserved3 = 0;
   StringRef Content;
   std::vector<RelocationInfo> Relocations;
 
-  Section(StringRef SegName, StringRef SectName)
-      : Segname(std::string(SegName)), Sectname(std::string(SectName)),
-        CanonicalName((Twine(SegName) + Twine(',') + SectName).str()) {}
-
-  Section(StringRef SegName, StringRef SectName, StringRef Content)
-      : Segname(std::string(SegName)), Sectname(std::string(SectName)),
-        CanonicalName((Twine(SegName) + Twine(',') + SectName).str()),
-        Content(Content) {}
+  Section(StringRef SegName, StringRef SectName);
+
+  Section(StringRef SegName, StringRef SectName, StringRef Content);
 
   MachO::SectionType getType() const {
     return static_cast<MachO::SectionType>(Flags & MachO::SECTION_TYPE);
   }
 
   bool isVirtualSection() const {
     return (getType() == MachO::S_ZEROFILL ||
             getType() == MachO::S_GB_ZEROFILL ||

llvm/lib/ObjCopy/MachO/MachOObject.cpp

 //===- MachOObject.cpp - Mach-O object file model ---------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "MachOObject.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include <unordered_set>
 
 using namespace llvm;
 using namespace llvm::objcopy::macho;
 
+Section::Section(StringRef SegName, StringRef SectName)
+    : Segname(std::string(SegName)), Sectname(std::string(SectName)),
+      CanonicalName((Twine(SegName) + Twine(',') + SectName).str()) {}
+
+Section::Section(StringRef SegName, StringRef SectName, StringRef Content)
+    : Segname(std::string(SegName)), Sectname(std::string(SectName)),
+      CanonicalName((Twine(SegName) + Twine(',') + SectName).str()),
+      Content(Content) {}
+
 const SymbolEntry *SymbolTable::getSymbolByIndex(uint32_t Index) const {
   assert(Index < Symbols.size() && "invalid symbol index");
   return Symbols[Index].get();
 }
 
 SymbolEntry *SymbolTable::getSymbolByIndex(uint32_t Index) {
   return const_cast<SymbolEntry *>(
       static_cast<const SymbolTable *>(this)->getSymbolByIndex(Index));

llvm/tools/llvm-ifs/ErrorCollector.h

 /// delete the ErrorCollector when some Errors have not been retrieved via
 /// makeError().
 ///
 //===-----------------------------------------------------------------------===/
 
 #ifndef LLVM_TOOLS_LLVM_IFS_ERRORCOLLECTOR_H
 #define LLVM_TOOLS_LLVM_IFS_ERRORCOLLECTOR_H
 
+#include "llvm/Support/Error.h"
 #include "llvm/Support/raw_ostream.h"
 #include <vector>
 
 namespace llvm {
 
-class Error;
-
 namespace ifs {
 
 class ErrorCollector {
 public:
   /// Upon destruction, an ErrorCollector will crash if UseFatalErrors=true and
   /// there are remaining errors that haven't been fetched by makeError().
   ErrorCollector(bool UseFatalErrors = true) : ErrorsAreFatal(UseFatalErrors) {}
   // Don't allow copying.