Next step toward making string diagnostics correct: handle

Next step toward making string diagnostics correct: handle
escapes in the string for subtoken positioning.  This gives
us working examples like:

t.m:5:16: warning: field width should have type 'int', but argument has type 'unsigned int'
  printf("\n\n%*d", (unsigned) 1, 1);
               ^    ~~~~~~~~~~~~

where before the caret pointed two spaces to the left.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@64940 91177308-0d34-0410-b5e6-96231b3b80d8