Pull Lexer's CharInfo table out for general use throughout Clang.

Rewriting the same predicates over and over again is bad for code size and
code maintainence. Using the functions in <ctype.h> is generally unsafe
unless they are specified to be locale-independent (i.e. only isdigit and
isxdigit).

The next commit will try to clean up uses of <ctype.h> functions within Clang.
StmtPrinter: Write large char values using \u or \U.

This may not always be valid, but we were previously just
emitting them raw.

While here, s/isprint/isPrintable/ (using the new CharInfo).
Simplify logic for avoiding concatenation after numeric constants.

I threw in a couple of test cases for UD-suffixes -- already working, but
it wasn't immediately obvious to me.
Excise <cctype> from Clang (except clang-tblgen) in favor of CharInfo.h.

Nearly all of these changes are one-to-one replacements; the few that
aren't have to do with custom identifier validation.