CommandLine: Unregister options in the destructor

[nhaehnle]

Summary

Fix a corruption issue when LLVM is used as a shared library in a plugin
setting.

Consider the case where A depends on B depends on LLVM, where B is a
shared compiler library built on LLVM that defines command-line options
(cl::opt) because it can also be used in a non-plugin setting.

If A is loaded and unloaded multiple times, causing B to be loaded and
unloaded multiple times while LLVM *isn't* unloaded, then corruption
results without this change: as B is unloaded, its cl::opt globals
disappear but the GlobalParser still holds a (dangling) reference to
them. Furthermore, if A and B are loaded a second time, the cl::opt
globals are constructed again which fails because options of the same
name are still registered.

The straightforward fix is to remove cl::opts from the GlobalParser in
the destructor.

[nhaehnle]

Abandoning this in favor of a stack of changes including D129129

llvm/include/llvm/Support/CommandLine.h

explicit Option(enum NumOccurrencesFlag OccurrencesFlag,
         HiddenFlag(Hidden), Formatting(NormalFormatting), Misc(0),
         FullyInitialized(false), Position(0), AdditionalVals(0) {
     Categories.push_back(&getGeneralCategory());
   }
 
   inline void setNumAdditionalVals(unsigned n) { AdditionalVals = n; }
 
 public:
-  virtual ~Option() = default;
+  virtual ~Option() {
+    if (FullyInitialized)
+      removeArgument();
+  }
 
   // Register this argument with the commandline system.
   //
   void addArgument();
 
   /// Unregisters this option from the CommandLine system.
   ///
-  /// This option must have been the last option registered.
-  /// For testing purposes only.
   void removeArgument();
 
   // Return the width of the option tag for printing...
   virtual size_t getOptionWidth() const = 0;
 
   // Print out information about this option. The to-be-maintained width is
   // specified.
   //

llvm/lib/Support/CommandLine.cpp

extrahelp::extrahelp(StringRef Help) : morehelp(Help) {
   GlobalParser->MoreHelp.push_back(Help);
 }
 
 void Option::addArgument() {
   GlobalParser->addOption(this);
   FullyInitialized = true;
 }
 
-void Option::removeArgument() { GlobalParser->removeOption(this); }
+void Option::removeArgument() {
+  // This method may be called from global destructors. Do nothing if the global
+  // parser has already been destroyed.
+  if (auto *Parser = GlobalParser.peek())
+    Parser->removeOption(this);
+}
 
 void Option::setArgStr(StringRef S) {
   if (FullyInitialized)
     GlobalParser->updateArgStr(this, S);
   assert((S.empty() || S[0] != '-') && "Option can't start with '-");
   ArgStr = S;
   if (ArgStr.size() == 1)
     setMiscFlag(Grouping);