Unterminated string literals should be rejected by the lexer

At the moment Wave accepts text like this:
```
"A
B"
```
which a user might accidentally write, intending to create a multi-line string literal with an embedded newline. In fact these "classic" (pre C++11) string literals [disallow newlines](https://en.cppreference.com/w/cpp/language/string_literal).
Unfortunately, Wave has been turning these into a series of tokens:
```
(#34 ) at /tmp/blah.c ( 1/ 1): >"<
IDENTIFIER (#380) at /tmp/blah.c ( 1/ 2): >A<
NEWLINE (#394) at /tmp/blah.c ( 1/ 3): >\n<
IDENTIFIER (#380) at /tmp/blah.c ( 2/ 1): >B<
(#34 ) at /tmp/blah.c ( 2/ 2): >"<
```
It can then go on to do work on these tokens, like directive evaluation, as described in bug #225.

Wave should produce a lexer exception in this case instead.