Fix assertions and wrong output from StmtPrinter's string literal printing.

String literals (including unicode ones) can contain non-Unicode codepoints
if they were written using \x or similar. Write those out using \x, but be
careful that the following character can't be misinterpreted as part of the
\x escape sequence. Convert UTF-16 surrogate pairs back to codepoints before
rendering them.

llvm-svn: 154069