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

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,67 @@
+//===-- 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 will have internal 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
----------------
AaronBallman wrote:

Would you mind trying an experiment for me? Would you add this to ASTMatchers.h and see how the numbers change for exported symbols:
```
extern const internal::VariadicDynCastAllOfMatcher<Decl, MSPropertyDecl>
    msPropertyDecl;

AST_MATCHER_P(MSPropertyDecl, hasGetter, std::string, Name) {
  return Node.hasGetter() && Node.getGetterId()->isStr(Name);
}
```
I'm asking because AST matchers often have a symbol explosion problem which is easy to overlook. If this adds two exported symbols, I don't worry to much. If it adds 20, it may be more worrying that we're already near 47k out of 65k (not that I think we're going to add that many matchers, just that I worry we'll add more symbols more quickly than we realize).

https://github.com/llvm/llvm-project/pull/108276