[Clang][Cygwin] Disable shared libs on Cygwin by default (llvm#138119)

This change follows MinGW decisions, otherwise build with GCC fail with:
```
FAILED: bin/msys-clang-cpp-21.0git.dll lib/libclang-cpp.dll.a
...
/usr/lib/gcc/x86_64-pc-msys/13.3.0/../../../../x86_64-pc-msys/bin/ld: error: export ordinal too large: 92250
```

Co-authored-by: jeremyd2019 <[email protected]>

Author: mati865

clang/tools/CMakeLists.txt

@@ -21,9 +21,10 @@ endif()
 add_clang_subdirectory(c-index-test)
 
 add_clang_subdirectory(clang-refactor)
-# For MinGW we only enable shared library if LLVM_LINK_LLVM_DYLIB=ON.
+# For MinGW/Cygwin we only enable shared library if LLVM_LINK_LLVM_DYLIB=ON.
 # Without that option resulting library is too close to 2^16 DLL exports limit.
-if(UNIX OR (MSVC AND LLVM_BUILD_LLVM_DYLIB_VIS) OR (MINGW AND LLVM_LINK_LLVM_DYLIB))
+if((UNIX AND NOT CYGWIN) OR (MSVC AND LLVM_BUILD_LLVM_DYLIB_VIS) OR
+  ((MINGW OR CYGWIN) AND LLVM_LINK_LLVM_DYLIB))
   add_clang_subdirectory(clang-shlib)
 endif()
 

clang/tools/libclang/CMakeLists.txt

@@ -111,7 +111,8 @@ if (LLVM_EXPORTED_SYMBOL_FILE)
                      DEPENDS ${LIBCLANG_VERSION_SCRIPT_FILE})
 endif()
 
-if(LLVM_ENABLE_PIC OR (WIN32 AND NOT LIBCLANG_BUILD_STATIC))
+if((NOT (WIN32 OR CYGWIN) AND LLVM_ENABLE_PIC) OR
+  ((WIN32 OR CYGWIN) AND NOT LIBCLANG_BUILD_STATIC))
   set(ENABLE_SHARED SHARED)
 endif()