C++: Set clang_version to the default from previous frontend version

This ensures that `__atomic_fetch_min` parses and that the number of
builtins does not changed compared to the previous version of the
frontend.

cpp/ql/test/library-tests/syntax-zoo/builtin.c

@@ -1,4 +1,4 @@
-// semmle-extractor-options: --clang
+// semmle-extractor-options: --clang --edg --clang_version --edg 30500
 
 #define vector(elcount, type)  __attribute__((vector_size((elcount)*sizeof(type)))) type