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

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

clang/tools/libclang/CMakeLists.txt

diff --git a/clang/tools/libclang/CMakeLists.txt b/clang/tools/libclang/CMakeLists.txt
index ac7a9a8..b6662b6 100644
--- a/clang/tools/libclang/CMakeLists.txt
+++ b/clang/tools/libclang/CMakeLists.txt
@@ -92,7 +92,7 @@
   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()