 [Mangler] Fix a bug in EncodeStringAsNumber.
 [Mangler] Fix a bug in EncodeStringAsNumber.

When we encode a string into an APInt the size of the APInt needs to be large
enough for the string (we got this part right), but also large enough for
arithmetic with the value of character length.

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift