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.c

@@ -0,0 +1,14 @@
+#define STR1(u) # u
+#define pass(a) a
+#define __ASM_REG(reg)         STR1(one##reg)
+#define _ASM_DX         __ASM_REG(tok)
+X162 pass(__ASM_REG(tok))
+X161 pass(_ASM_DX)
+X163 pass(STR1(one##tok))
+
+X170 pass(x ## y)
+X171 pass(x pass(##) y)
+
+#define Y(x) Z(x)
+#define X Y
+X180 return X(X(1));

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));