Revert "[Cygwin] Define LLVM_ABI for Cygwin (llvm#143222)"

Flang Merge CI rejects changes to llvm/Support/Compiler.h because precompiled
headers aren't rebuilt when their dependencies change.

This reverts commit 60d0004.

Author: kikairoya

llvm/include/llvm/Support/Compiler.h

@@ -209,7 +209,7 @@
 #define LLVM_ABI_FRIEND LLVM_ABI
 #define LLVM_ABI_EXPORT __declspec(dllexport)
 #elif defined(__ELF__) || defined(__MINGW32__) || defined(_AIX) ||             \
-    defined(__MVS__) || defined(__CYGWIN__)
+    defined(__MVS__)
 #define LLVM_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define LLVM_ABI_FRIEND
 #define LLVM_TEMPLATE_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT