accurate diagnostics.  For test/Lexer/comments.c we now emit:

int x = 000000080;  /* expected-error {{invalid digit}} */
               ^
constants.c:7:4: error: invalid digit '8' in octal constant
00080;             /* expected-error {{invalid digit}} */
   ^


The last line is due to an escaped newline.  The full line looks like:

int y = 0000\
00080;             /* expected-error {{invalid digit}} */


Previously, we emitted:
constants.c:4:9: error: invalid digit '8' in octal constant
int x = 000000080;  /* expected-error {{invalid digit}} */
        ^
constants.c:6:9: error: invalid digit '8' in octal constant
int y = 0000\
        ^

which isn't too bad, but the new way is better for the user,
regardless of whether there is an escaped newline or not.

All the other lexer-related diagnostics should switch over 
to using AdvanceToTokenCharacter where appropriate.  Help
wanted :).

This implements test/Lexer/constants.c.

llvm-svn: 39906