[Lex] Stop allocating/deallocating MacroInfo on a linked list. NFC

[sammccall]

Summary

This list was originally used for to make sure MacroInfo's clever memory
management got called (1f1e4bdbf7815c), but that was
simplified in 73a29662b9bf640a and 1f1e4bdbf7815c, and there's nothing left.

[sammccall]

(I'm sure this doesn't matter much, I just ran into it while trying to understand lifetimes of various macro description objects)

clang/include/clang/Lex/MacroInfo.h

class MacroInfo {
   bool IsAllowRedefinitionsWithoutWarning : 1;
 
   /// Must warn if the macro is unused at the end of translation unit.
   bool IsWarnIfUnused : 1;
 
   /// Whether this macro was used as header guard.
   bool UsedForHeaderGuard : 1;
 
-  // Only the Preprocessor gets to create and destroy these.
+  // Only the Preprocessor gets to create these.
   MacroInfo(SourceLocation DefLoc);
-  ~MacroInfo() = default;
 
 public:
   /// Return the location that the macro was defined at.
   SourceLocation getDefinitionLoc() const { return Location; }
 
   /// Set the location of the last token in the macro.
   void setDefinitionEndLoc(SourceLocation EndLoc) { EndLocation = EndLoc; }
 

clang/include/clang/Lex/Preprocessor.h

private:
 
   /// Stack of backtrack positions, allowing nested backtracks.
   ///
   /// The EnableBacktrackAtThisPos() method pushes a position to
   /// indicate where CachedLexPos should be set when the BackTrack() method is
   /// invoked (at which point the last position is popped).
   std::vector<CachedTokensTy::size_type> BacktrackPositions;
 
-  struct MacroInfoChain {
-    MacroInfo MI;
-    MacroInfoChain *Next;
-  };
-
-  /// MacroInfos are managed as a chain for easy disposal.  This is the head
-  /// of that list.
-  MacroInfoChain *MIChainHead = nullptr;
-
   /// True if \p Preprocessor::SkipExcludedConditionalBlock() is running.
   /// This is used to guard against calling this function recursively.
   ///
   /// See comments at the use-site for more context about why it is needed.
   bool SkippingExcludedConditionalBlock = false;
 
   /// Keeps track of skipped range mappings that were recorded while skipping
   /// excluded conditional directives. It maps the source buffer pointer at

clang/lib/Lex/PPDirectives.cpp

 
 using namespace clang;
 
 //===----------------------------------------------------------------------===//
 // Utility Methods for Preprocessor Directive Handling.
 //===----------------------------------------------------------------------===//
 
 MacroInfo *Preprocessor::AllocateMacroInfo(SourceLocation L) {
-  auto *MIChain = new (BP) MacroInfoChain{L, MIChainHead};
-  MIChainHead = MIChain;
-  return &MIChain->MI;
+  static_assert(std::is_trivially_destructible_v<MacroInfo>, "");
+  return new (BP) MacroInfo(L);
 }
 
 DefMacroDirective *Preprocessor::AllocateDefMacroDirective(MacroInfo *MI,
                                                            SourceLocation Loc) {
   return new (BP) DefMacroDirective(MI, Loc);
 }
 
 UndefMacroDirective *

clang/lib/Lex/Preprocessor.cpp

Preprocessor::Preprocessor(std::shared_ptr<PreprocessorOptions> PPOpts,
   MaxTokens = LangOpts.MaxTokens;
 }
 
 Preprocessor::~Preprocessor() {
   assert(BacktrackPositions.empty() && "EnableBacktrack/Backtrack imbalance!");
 
   IncludeMacroStack.clear();
 
-  // Destroy any macro definitions.
-  while (MacroInfoChain *I = MIChainHead) {
-    MIChainHead = I->Next;
-    I->~MacroInfoChain();
-  }
-
   // Free any cached macro expanders.
   // This populates MacroArgCache, so all TokenLexers need to be destroyed
   // before the code below that frees up the MacroArgCache list.
   std::fill(TokenLexerCache, TokenLexerCache + NumCachedTokenLexers, nullptr);
   CurTokenLexer.reset();
 
   // Free any cached MacroArgs.
   for (MacroArgs *ArgList = MacroArgCache; ArgList;)