Revert change to handling "#@" in SWIG preprocessor

The extra flexibility in expanding "#@" was considered inappropriate,
see the discussion in #2096, so revert the preprocessor part of the
changes of 5bf1497d7 (Allow customizing type mangling in SWIG
preprocessor, 2021-11-10).

This unfortunately means that the macros used in <attribute.i> don't
work for C backend any longer, so a previously passed unit test had to
be disabled.

Examples/test-suite/c/Makefile.in

@@ -56,6 +56,7 @@ FAILING_CPP_TESTS := \
     grouping \
     import_nomodule \
     li_attribute \
+    li_attribute_template \
     li_boost_shared_ptr_attribute \
     li_std_deque \
     li_std_wstring \