enable plugins for clang-tidy

Fixes https://bugs.llvm.org//show_bug.cgi?id=32739, so pinging people who previously discussed that issue. This takes advantage of the existing plugin capabilities of llvm to add support for `-load` simply by requesting adding the header file to request it. I've tested out-of-tree something of this form (leaving out a few details of the check logic itself), and confirmed that it seems to load and get called with this patch:

```
#include "clang/AST/ASTContext.h"
#include "clang/ASTMatchers/ASTMatchFinder.h"
#include "clang-tidy/ClangTidy.h"
#include "clang-tidy/ClangTidyCheck.h"
#include "clang-tidy/ClangTidyModule.h"
#include "clang-tidy/ClangTidyModuleRegistry.h"

using namespace clang;
using namespace clang::tidy;
using namespace clang::ast_matchers;

class MyTestCheck : public ClangTidyCheck;

namespace {
class CTTestModule : public ClangTidyModule {
public:
  void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
    CheckFactories.registerCheck<MyTestCheck>("mytest");
  }
};
} // namespace

namespace clang {
namespace tidy {

// Register the CTTestTidyModule using this statically initialized variable.
static ClangTidyModuleRegistry::Add<::CTTestModule>
    X("mytest-module", "Adds my checks.");

// This anchor is used to force the linker to link in the generated object file
// and thus register the CTTestModule.
volatile int CTTestModuleAnchorSource = 0;

} // namespace tidy
} // namespace clang
```
```
$ ./bin/clang-tidy --checks=-*,mytest -list-checks -load mytestcheckplugin.so
Enabled checks:
    mytest
```

Differential Revision: https://reviews.llvm.org/D111100

Author: vtjnash

clang-tools-extra/clang-tidy/tool/CMakeLists.txt

@@ -29,11 +29,17 @@ clang_target_link_libraries(clangTidyMain
   clangToolingCore
   )
 
+# Support plugins.
+if(CLANG_PLUGIN_SUPPORT)
+  set(support_plugins SUPPORT_PLUGINS)
+endif()
+
 add_clang_tool(clang-tidy
   ClangTidyToolMain.cpp
-  )
-add_dependencies(clang-tidy
+
+  DEPENDS
   clang-resource-headers
+  ${support_plugins}
   )
 clang_target_link_libraries(clang-tidy
   PRIVATE
@@ -50,6 +56,9 @@ target_link_libraries(clang-tidy
   ${ALL_CLANG_TIDY_CHECKS}
   )
 
+if(CLANG_PLUGIN_SUPPORT)
+  export_executable_symbols_for_plugins(clang-tidy)
+endif()
 
 install(PROGRAMS clang-tidy-diff.py
   DESTINATION share/clang

clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp

@@ -20,6 +20,7 @@
 #include "../GlobList.h"
 #include "clang/Tooling/CommonOptionsParser.h"
 #include "llvm/Support/InitLLVM.h"
+#include "llvm/Support/PluginLoader.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/TargetSelect.h"