Export LLVM plugin entrypoint for windows shared library builds

Add Symbol visibility macro to llvmGetPassPluginInfo so plugins export it for
windows shared library builds with explicit symbol visibility macros enabled.
This avoids the need for existing plugins having to manually add visibility
macros to there llvmGetPassPluginInfo function to work on windows.

This is part of the work to enable LLVM_BUILD_LLVM_DYLIB and plugins on window.

Author: fsfod

llvm/include/llvm/Passes/PassPlugin.h

@@ -107,7 +107,7 @@ class PassPlugin {
 ///   };
 /// }
 /// ```
-extern "C" ::llvm::PassPluginLibraryInfo LLVM_ATTRIBUTE_WEAK
+extern "C" ::llvm::PassPluginLibraryInfo LLVM_ATTRIBUTE_WEAK LLVM_ABI_EXPORT
 llvmGetPassPluginInfo();
 
 #endif /* LLVM_PASSES_PASSPLUGIN_H */