use the full spelling of a string literal token so that trigraphs

and escaped newlines don't throw off the offset computation.

On this testcase:
  printf("abc\
def"
         "%*d", (unsigned) 1, 1);

Before:
t.m:5:5: warning: field width should have type 'int', but argument has type 'unsigned int'
def"
    ^

after:
t.m:6:12: warning: field width should have type 'int', but argument has type 'unsigned int'
         "%*d", (unsigned) 1, 1);
           ^    ~~~~~~~~~~~~

llvm-svn: 64930