[Demangler] Fix a small issue with symbolic references in mangled names.

Found by inspection.

Author: DougGregor

lib/Demangling/Demangler.cpp

@@ -136,7 +136,7 @@ swift::Demangle::makeSymbolicMangledNameStringRef(const char *base) {
     // Skip over symbolic references.
     if (*end >= '\x01' && *end <= '\x17')
       end += sizeof(uint32_t);
-    if (*end >= '\x18' && *end <= '\x1F')
+    else if (*end >= '\x18' && *end <= '\x1F')
       end += sizeof(void*);
     ++end;
   }