Allow StmtPrinter to print signed character literals. Given the code:
char c = '\200';
The character literal is presently printed as:
char c = '\Uffffff80';
The original literal has the signed value -128 (or unsigned value 128)
and only has any meaning when clamped to the extended ASCII range.
'\Uffffff80' isn't a valid character.
With the patch, the literal is printed as:
char c = '\x80';
As a side note, this doesn't handle multicharacter literals, but I
don't think there is enough information in the AST to make that
possible. They are implementation defined, anyway.