[libclang][Cygwin] Use LLVM_EXPORTED_SYMBOL_FILE (*.def file) for Cygwin (llvm#147278)

This is not mandatory but recommended for completeness and consistency
with MinGW.

Author: kikairoya

clang/tools/libclang/CMakeLists.txt

@@ -97,7 +97,7 @@ if(MSVC)
   set(LLVM_EXPORTED_SYMBOL_FILE)
 endif()
 
-if (UNIX AND NOT APPLE AND NOT ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
+if (UNIX AND NOT APPLE AND NOT ${CMAKE_SYSTEM_NAME} MATCHES "AIX" AND NOT CYGWIN)
   set(LLVM_EXPORTED_SYMBOL_FILE)
   set(USE_VERSION_SCRIPT ${LLVM_HAVE_LINK_VERSION_SCRIPT})
 endif()