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

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

This is [not legal](https://timsong-cpp.github.io/cppwp/n4868/vector#overview-4)
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.

Reviewed By: saugustine, MaskRay

Differential Revision: https://reviews.llvm.org/D135906

Author: angelomatni

llvm/lib/ObjCopy/MachO/MachOObject.cpp

@@ -13,6 +13,15 @@
 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();

llvm/lib/ObjCopy/MachO/MachOObject.h

@@ -57,14 +57,9 @@ struct Section {
   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);

llvm/tools/llvm-ifs/ErrorCollector.h

@@ -21,13 +21,12 @@
 #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 {