[Windows] Fix plugin registry symbols not exported/linked with CLANG_LINK_CLANG_DYLIB

When building LLVM with CLANG_LINK_CLANG_DYLIB=ON on Windows, external
plugins cannot register through llvm::Registry<clang::PluginASTAction>
due to two issues:

1. Static data members (Head, Tail) are filtered out during symbol export
2. Static methods (add_node(), begin()) are not linked into the executable

This change fixes both issues:

- Modified extract_symbols.py to recognize and export Registry<T>::Head
  and Registry<T>::Tail static members using pattern matching instead of
  filtering them out as non-function symbols.

- Added code in driver.cpp that references Registry methods, forcing the
  linker to include these symbols in clang.exe even though they're not
  directly called by the driver.

Fixes #163367

Author: zond

clang/tools/driver/driver.cpp

@@ -55,6 +55,25 @@
 #include <set>
 #include <system_error>
 
+#if defined(CLANG_PLUGIN_SUPPORT) && defined(_WIN32)
+#include "clang/Frontend/FrontendPluginRegistry.h"
+
+// Force plugin registry symbols into clang.exe on Windows so plugins can
+// register. These methods exist in libraries but aren't linked by default
+// because they're unreferenced. Taking their addresses forces the linker to
+// include them.
+namespace {
+void ForcePluginRegistrySymbols() {
+  using PluginRegistry = llvm::Registry<clang::PluginASTAction>;
+  // Use volatile to prevent the compiler from optimizing away these references
+  volatile auto add_node_ptr = &PluginRegistry::add_node;
+  volatile auto begin_ptr = &PluginRegistry::begin;
+  (void)add_node_ptr;
+  (void)begin_ptr;
+}
+} // anonymous namespace
+#endif
+
 using namespace clang;
 using namespace clang::driver;
 using namespace llvm::opt;

llvm/utils/extract_symbols.py

@@ -105,6 +105,11 @@ def should_keep_microsoft_symbol(symbol, calling_convention_decoration):
     # Skip X86GenMnemonicTables functions, they are not exposed from llvm/include/.
     elif re.match(r"\?is[A-Z0-9]*@X86@llvm", symbol):
         return None
+    # Keep Registry<T>::Head and Registry<T>::Tail static members for plugin support.
+    # Pattern matches: ?Head@?$Registry@<template_args>@llvm@@ or ?Tail@?$Registry@...
+    elif ("?$Registry@" in symbol and "@llvm@@" in symbol and
+          (symbol.startswith("?Head@") or symbol.startswith("?Tail@"))):
+        return symbol
     # Keep mangled llvm:: and clang:: function symbols. How we detect these is a
     # bit of a mess and imprecise, but that avoids having to completely demangle
     # the symbol name. The outermost namespace is at the end of the identifier