swift234

C/C++/ObjC Interop: string/char escapes

If we're going to import C/C++/ObjC code, we ought to "just work" with
their escape patterns when reasonable. (No error-prone octal escapes or
trigraph support). Also, update LangRef to document what the escapes do.

This patch DOES fix a bug with lexing operators in the case of (already
warned about) embedded NUL bytes within a source file.

This patch DOES NOT change what we consider to be valid whitespace in
the language.

Swift SVN r3970