`libTestPlugin.dylib` dynamic library was previously linking against
`libLLVMSupport.a`, which is already linked into the Swift compiler
binary. This caused multiple conflicting definitions of `LLVMSupport`
lib symbols, leading to ODR violations. This issue has been addressed
by linking against `libLLVMSupport` via `-hidden-lLLVMSupport` flag,
ensuring `libLLVMSupport` symbols remain hidden within the plugin,
preventing conflicts with those in the Swift compiler.
Fixes: https://github.com/swiftlang/swift/issues/77771.