[PR 95149] reimplement raw literal lexing

[Nathan Sidwell]

pr95149 is a false positive static analysis checker.  But it encouranged 
me to fix raw string lexing, which does contain a complicated macro and 
pointers to local variables.  The reimplementation does away with that 
macro.  Part of the complication is we need to undo some of the fresh 
line processing -- trigraph notes and escaped line continuations.  But 
the undone characters need to go through the raw string processing, as 
they can legitimately be part of the prefix marker.  however, in this 
reformulation we only process one line marker at a time[*], so there's a 
limited number of undone characters.  We can arrange the buffering to 
make sure we don't split such an append sequence, and then simply take 
the characters from the append buffer.

The prefix scanner had a switch statement, which I discovered was not 
optimized as well as an if of a bunch of explicit comparisons (pr 95208 
filed).

Finally I adjusted the failure mode.  When we get a bad prefix, we lex 
up until the next '"', thus often swallowing the whole raw string. 
Previously we'd bail and then the lexer would usually generate stupid 
tokens, particularly when meeting the ending '"'.

pushed to master

-- 
Nathan Sidwell
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