Fix undefined __USER_LABEL_PREFIX compiling swift/LLVMPasses

Author: hughbe

include/swift/Runtime/Config.h

@@ -232,8 +232,13 @@
 #endif
 
 #if !defined(__USER_LABEL_PREFIX__)
+// MSVC doesn't define __USER_LABEL_PREFIX.
+#if defined(_MSC_VER)
+#define __USER_LABEL_PREFIX__
+#else
 #error __USER_LABEL_PREFIX__ is undefined
 #endif
+#endif
 
 // Workaround the bug of clang in Cygwin 64bit
 // https://llvm.org/bugs/show_bug.cgi?id=26744