[Clang] Add explicit visibility symbol macros (llvm#108276)

This is part of the effort to support for enabling plugins on windows by
adding better support for building llvm and clang as a DLL. These macros
are similar to the ones i added in llvm#96630, but are for clang.
Added explicit symbol visibility macros definitions that are defined in
a new header and will be used to for shared library builds of clang to
export
symbols. 
Updated clang cmake to define a macro to enable the symbol visibility
macros and explicitly disable them for clang tools that always use
static linking.

---------

Co-authored-by: Aaron Ballman <aaron@aaronballman.com>

clang/cmake/modules/AddClang.cmake

@@ -108,6 +108,17 @@ macro(add_clang_library name)
   endif()
   llvm_add_library(${name} ${LIBTYPE} ${ARG_UNPARSED_ARGUMENTS} ${srcs})
 
+  if(MSVC AND NOT CLANG_LINK_CLANG_DYLIB)
+    # Make sure all consumers also turn off visibility macros so there not trying to dllimport symbols.
+    target_compile_definitions(${name} PUBLIC CLANG_BUILD_STATIC)
+    if(TARGET "obj.${name}")
+      target_compile_definitions("obj.${name}" PUBLIC CLANG_BUILD_STATIC)
+    endif()
+  elseif(NOT ARG_SHARED AND NOT ARG_STATIC)
+    # Clang component libraries linked in to clang-cpp are declared without SHARED or STATIC
+    target_compile_definitions("obj.${name}" PUBLIC CLANG_EXPORTS)
+  endif()
+
   set(libs ${name})
   if(ARG_SHARED AND ARG_STATIC)
     list(APPEND libs ${name}_static)

clang/include/clang/Support/Compiler.h

@@ -0,0 +1,63 @@
+//===-- clang/Support/Compiler.h - Compiler abstraction support -*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines explicit visibility macros used to export symbols from
+// clang-cpp
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef CLANG_SUPPORT_COMPILER_H
+#define CLANG_SUPPORT_COMPILER_H
+
+#include "llvm/Support/Compiler.h"
+
+/// CLANG_ABI is the main export/visibility macro to mark something as
+/// explicitly exported when clang is built as a shared library with everything
+/// else that is unannotated having hidden visibility.
+///
+/// CLANG_EXPORT_TEMPLATE is used on explicit template instantiations in source
+/// files that were declared extern in a header. This macro is only set as a
+/// compiler export attribute on windows, on other platforms it does nothing.
+///
+/// CLANG_TEMPLATE_ABI is for annotating extern template declarations in headers
+/// for both functions and classes. On windows its turned in to dllimport for
+/// library consumers, for other platforms its a default visibility attribute.
+#ifndef CLANG_ABI_GENERATING_ANNOTATIONS
+// Marker to add to classes or functions in public headers that should not have
+// export macros added to them by the clang tool
+#define CLANG_ABI_NOT_EXPORTED
+// Some libraries like those for tablegen are linked in to tools that used
+// in the build so can't depend on the llvm shared library. If export macros
+// were left enabled when building these we would get duplicate or
+// missing symbol linker errors on windows.
+#if defined(CLANG_BUILD_STATIC)
+#define CLANG_ABI
+#define CLANG_TEMPLATE_ABI
+#define CLANG_EXPORT_TEMPLATE
+#elif defined(_WIN32) && !defined(__MINGW32__)
+#if defined(CLANG_EXPORTS)
+#define CLANG_ABI __declspec(dllexport)
+#define CLANG_TEMPLATE_ABI
+#define CLANG_EXPORT_TEMPLATE __declspec(dllexport)
+#else
+#define CLANG_ABI __declspec(dllimport)
+#define CLANG_TEMPLATE_ABI __declspec(dllimport)
+#define CLANG_EXPORT_TEMPLATE
+#endif
+#elif defined(__ELF__) || defined(__MINGW32__) || defined(_AIX)
+#define CLANG_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
+#define CLANG_TEMPLATE_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
+#define CLANG_EXPORT_TEMPLATE
+#elif defined(__MACH__) || defined(__WASM__)
+#define CLANG_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
+#define CLANG_TEMPLATE_ABI
+#define CLANG_EXPORT_TEMPLATE
+#endif
+#endif
+
+#endif

clang/tools/libclang/CMakeLists.txt

@@ -167,6 +167,10 @@ if(ENABLE_SHARED)
       PROPERTIES
       VERSION ${LIBCLANG_LIBRARY_VERSION}
       DEFINE_SYMBOL _CINDEX_LIB_)
+      # Avoid declaring clang c++ symbols that are statically linked into libclang as dllimport'ed.
+      # If llvm/libclang-cpp dll is also being built for windows clang c++ symbols will still be
+      # implicitly be exported from libclang.
+      target_compile_definitions(libclang PRIVATE CLANG_BUILD_STATIC)
   elseif(APPLE)
     set(LIBCLANG_LINK_FLAGS " -Wl,-compatibility_version -Wl,1")
     set(LIBCLANG_LINK_FLAGS "${LIBCLANG_LINK_FLAGS} -Wl,-current_version -Wl,${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}")