Fix a preprocessor expression evaluation bug

A subexpression in parentheses lost its string/int type flag and
instead used whatever type was left in the stack entry from
previous use.  In practice we mostly got away with this because
most preprocessor expressions are integer, but it could have
resulted in a preprocessor expression incorrectly evaluating as
zero.  If -Wextra was in use you got a warning:

Warning 202: Error: 'Can't mix strings and integers in expression'

Fixes #1384

Examples/test-suite/preproc.i

@@ -11,6 +11,22 @@
 #pragma SWIG nowarn=890                                      /* lots of Go name conflicts */
 #pragma SWIG nowarn=206                                      /* Unexpected tokens after #endif directive. */
 
+/* Regression test: in SWIG < 4.1.0 this triggered the two #error cases.
+ * Github issue #1384
+ */
+#if "" != ""
+#endif
+#if 1 && (!0)
+// Should go here
+#else
+# error BUG
+#endif
+#if ((("" == ""))) || (1 && (!1))
+// Should go here
+#else
+# error BUG
+#endif
+
 %{
 #if defined(__clang__)
 /*Suppress: warning: use of logical '&&' with constant operand [-Wconstant-logical-operand]*/