[libclang] Add missing dllexport annotation (#147108)

All other declarations of clang-c already have CINDEX_LINKAGE.

This missing annotation causes a linker error when building
`tools/clang/unittests/libclang/CrashTests/libclangCrashTests.exe` for
the Cygwin target.
On the regular Win32 target, this issue went unnoticed because the
entire libclang gtest-based testsuite is currently disabled for that
platform.

Author: kikairoya

clang/include/clang-c/FatalErrorHandler.h

@@ -11,21 +11,22 @@
 #define LLVM_CLANG_C_FATAL_ERROR_HANDLER_H
 
 #include "clang-c/ExternC.h"
+#include "clang-c/Platform.h"
 
 LLVM_CLANG_C_EXTERN_C_BEGIN
 
 /**
  * Installs error handler that prints error message to stderr and calls abort().
  * Replaces currently installed error handler (if any).
  */
-void clang_install_aborting_llvm_fatal_error_handler(void);
+CINDEX_LINKAGE void clang_install_aborting_llvm_fatal_error_handler(void);
 
 /**
  * Removes currently installed error handler (if any).
  * If no error handler is intalled, the default strategy is to print error
  * message to stderr and call exit(1).
  */
-void clang_uninstall_llvm_fatal_error_handler(void);
+CINDEX_LINKAGE void clang_uninstall_llvm_fatal_error_handler(void);
 
 LLVM_CLANG_C_EXTERN_C_END