Better cycle detection

[jyn514]

See http://port70.net/~nsz/c/c11/n1570.html#6.10.3.4, especially

2 If the name of the macro being replaced is found during this scan of the replacement list (not including the rest of the source file's preprocessing tokens), it is not replaced. Furthermore, if any nested replacements encounter the name of the macro being replaced, it is not replaced. These nonreplaced macro name preprocessing tokens are no longer available for further replacement even if they are later (re)examined in contexts in which that macro name preprocessing token would otherwise have been replaced.

Examples of cycles that should be detected:

// from real code in <signal.h> !!
#define sa_handler   __sa_handler.sa_handler
#define sa_sigaction __sa_handler.sa_sigaction
int main() {
    struct sigaction handler;
    handler.sa_handler = 0;
}

See also #5

[jyn514]

Proposed algorithm:

In https://github.com/jyn514/rcc/blob/master/src/lex/cpp.rs#L523, instead of always adding a token to self.pending, decide whether or not the token should undergo further replacement. If not, add a CppToken::NoReplacement to self.pending; otherwise add CppToken::Token