In the discussion of the fallout from the enhancement for pr71625
it was pointed out that STRING_CSts in Gimple dumps extend only
to the first nul and don't include any subsequent characters,
and that this makes the dumps harder to read and might give rise
to the question whether the code is correct.

In the attached patch I enhance the pretty_print_string() function
to print the full contents of a STRING_CST, including any embedded
nuls to make the dumps clearer.  I got rid of the single digit
escapes like '\1' since they make the string look ambiguous.
If TREE_STRING_LENGTH (node) is guaranteed to be non-zero these
days the test for it being so may be redundant but I figured it's
better to be safe than sorry.

A further enhancement might be to also distinguish the type of
the STRING_CST.

Martin