tccpp: Fix token pasting

See testcase.  We must always paste tokens (at least if not
currently substing a normal argument, which is a speed optimization
only now) but at the same time must not regard a ## token
coming from argument expansion as the token-paste operator, nor
if we constructed a ## token due to pasting itself (that was already
checked by pp/01.c).

tests/pp/17.expect

@@ -0,0 +1,6 @@
+X162 "onetok"
+X161 "onetok"
+X163 "one##tok"
+X170 x ## y
+X171 x ## y
+X180 return Z(Z(1));