[clang] [Clang][Cygwin] Fix symbol visibility definition (PR #138118)

[Mateusz Mikuła via cfe-commits]

https://github.com/mati865 created https://github.com/llvm/llvm-project/pull/138118

Currently building for Cygwin hits this error:
```
In file included from /h/projects/llvm-project/clang/lib/Basic/Attributes.cpp:17:
/h/projects/llvm-project/clang/include/clang/Basic/ParsedAttrInfo.h:180:73: error: invalid declarator before ‘;’ token
  180 | extern template class CLANG_TEMPLATE_ABI Registry<clang::ParsedAttrInfo>;
```
That's because `CLANG_TEMPLATE_ABI` ends up not being defined. The solution here is to follow MinGW case.

Split out from https://github.com/llvm/llvm-project/pull/134494

>From 41a22fcdb5d1c62ca47825039230c3dc03ccaf5e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Miku=C5=82a?= <oss at mateuszmikula.dev>
Date: Sat, 8 Feb 2025 13:57:49 +0100
Subject: [PATCH] [Clang][Cygwin] Fix symbol visibility definition
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Currently building for Cygwin hits this error:
```
In file included from /h/projects/llvm-project/clang/lib/Basic/Attributes.cpp:17:
/h/projects/llvm-project/clang/include/clang/Basic/ParsedAttrInfo.h:180:73: error: invalid declarator before ‘;’ token
  180 | extern template class CLANG_TEMPLATE_ABI Registry<clang::ParsedAttrInfo>;
```
That's because `CLANG_TEMPLATE_ABI` ends up not being defined.
The solution here is to follow MinGW case.
---
 clang/include/clang/Support/Compiler.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang/include/clang/Support/Compiler.h b/clang/include/clang/Support/Compiler.h
index 5a74f8e3b6723..e1ae3eda4ccc2 100644
--- a/clang/include/clang/Support/Compiler.h
+++ b/clang/include/clang/Support/Compiler.h
@@ -50,7 +50,7 @@
 #define CLANG_EXPORT_TEMPLATE
 #endif
 #elif defined(__ELF__) || defined(__MINGW32__) || defined(_AIX) ||             \
-    defined(__MVS__)
+    defined(__MVS__) || defined(__CYGWIN__)
 #define CLANG_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define CLANG_TEMPLATE_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define CLANG_EXPORT_TEMPLATE