[clang] [libclang][Cygwin] Use __declspec(dllexport) for libclang on Cygwin (PR #147122)

[Tomohiro Kashiwada via cfe-commits]

kikairoya wrote:

`git clang-format` makes reformat below, should it be applied?
```diff
diff --git a/clang/include/clang-c/Platform.h b/clang/include/clang-c/Platform.h
index 8d341ddd6f8e..1c28ccf81981 100644
--- a/clang/include/clang-c/Platform.h
+++ b/clang/include/clang-c/Platform.h
@@ -23,13 +23,13 @@ LLVM_CLANG_C_EXTERN_C_BEGIN
   #define CINDEX_EXPORTS
 #endif
 #if defined(_WIN32) || defined(__CYGWIN__)
-  #ifdef CINDEX_EXPORTS
-    #ifdef _CINDEX_LIB_
-      #define CINDEX_LINKAGE __declspec(dllexport)
-    #else
-      #define CINDEX_LINKAGE __declspec(dllimport)
-    #endif
-  #endif
+#ifdef CINDEX_EXPORTS
+#ifdef _CINDEX_LIB_
+#define CINDEX_LINKAGE __declspec(dllexport)
+#else
+#define CINDEX_LINKAGE __declspec(dllimport)
+#endif
+#endif
 #elif defined(CINDEX_EXPORTS) && defined(__GNUC__)
   #define CINDEX_LINKAGE __attribute__((visibility("default")))
 #endif
```

https://github.com/llvm/llvm-project/pull/147122