[clangd] Add library for clangd main function

The diff adds a library and header for clangd main function. That change allows to create custom builds for clangd outside the main LLVM repo. The diff also allows to use build system different from CMake to build clangd. The main reason for such change is an ability to use custom clang-tidy modules (created outside LLVM repo).

Test Plan:
```
ninja clangd
```
also check that necessary libs are installed aka
```
ninja install
...
ls <install folder>/lib/libclangdMain.a
```

Differential Revision: https://reviews.llvm.org/D145302

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

@@ -1,6 +1,13 @@
-add_clang_tool(clangd
+# Needed by LLVM's CMake checks because this file defines multiple targets.
+set(LLVM_OPTIONAL_SOURCES ClangdToolMain.cpp)
+
+add_clang_library(clangdMain
   ClangdMain.cpp
   Check.cpp
+  )
+
+add_clang_tool(clangd
+  ClangdToolMain.cpp
   $<TARGET_OBJECTS:obj.clangDaemonTweaks>
   )
 
@@ -13,7 +20,7 @@ if(CLANGD_BUILD_XPC)
   list(APPEND CLANGD_XPC_LIBS "clangdXpcJsonConversions" "clangdXpcTransport")
 endif()
 
-clang_target_link_libraries(clangd
+clang_target_link_libraries(clangdMain
   PRIVATE
   clangAST
   clangBasic
@@ -25,14 +32,14 @@ clang_target_link_libraries(clangd
   clangToolingCore
   clangToolingRefactoring
   clangToolingSyntax
-  )
-
-target_link_libraries(clangd
-  PRIVATE
   clangTidy
-
   clangDaemon
   clangdRemoteIndex
   clangdSupport
   ${CLANGD_XPC_LIBS}
   )
+
+clang_target_link_libraries(clangd
+  PRIVATE
+  clangdMain
+  )

clang-tools-extra/clangd/tool/ClangdMain.cpp

@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "ClangdMain.h"
 #include "ClangdLSPServer.h"
 #include "CodeComplete.h"
 #include "Compiler.h"
@@ -710,19 +711,14 @@ class FlagsConfigProvider : public config::Provider {
   }
 };
 } // namespace
-} // namespace clangd
-} // namespace clang
 
 enum class ErrorResultCode : int {
   NoShutdownRequest = 1,
   CantRunAsXPCService = 2,
   CheckFailed = 3
 };
 
-int main(int argc, char *argv[]) {
-  using namespace clang;
-  using namespace clang::clangd;
-
+int clangdMain(int argc, char *argv[]) {
   llvm::InitializeAllTargetInfos();
   llvm::sys::PrintStackTraceOnErrorSignal(argv[0]);
   llvm::sys::AddSignalHandler(
@@ -1041,3 +1037,6 @@ clangd accepts flags on the commandline, and in the CLANGD_FLAGS environment var
 
   return ExitCode;
 }
+
+} // namespace clangd
+} // namespace clang

clang-tools-extra/clangd/tool/ClangdMain.h

@@ -0,0 +1,19 @@
+//===--- ClangdMain.h - clangd main function ------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_TOOL_CLANGDMAIN_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_TOOL_CLANGDMAIN_H
+
+namespace clang {
+namespace clangd {
+// clangd main function (clangd server loop)
+int clangdMain(int argc, char *argv[]);
+} // namespace clangd
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANGD_TOOL_CLANGDMAIN_H

clang-tools-extra/clangd/tool/ClangdToolMain.cpp

@@ -0,0 +1,13 @@
+//===--- ClangdToolMain.cpp - clangd main function ------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "ClangdMain.h"
+
+int main(int argc, char **argv) {
+  return clang::clangd::clangdMain(argc, argv);
+}