lint: concatenate string literals from left to right

Previously, the string literals "1" "2" "3" "4" were concatenated in the
order "23", "234", "1234".  This influenced the location of the
diagnostics for traditional C (which doesn't know concatenation at all)
and for mixing regular strings and wide strings.

Now the diagnostics occur exactly where they are expected.  The first
string literal defines whether the whole string is regular or wide, and
any further string literals must match it.

In traditional C mode, there are more diagnostics than before, but that
doesn't hurt since they are still correct and nobody uses lint in
traditional C mode anyway.

(rillig)