[PATCH] Fix PR C++/36607 Incorrect type diagnostic on substracting casted char pointers

[Andrew Pinski]

Hi,
  The problem here is that fold will fold NOP_EXPR<INT, a +
OFFSET_TYPE_var> - NOP_EXPR<INT, a> into NOP_EXPR <INT,
OFFSET_TYPE_var> and then when converting (via convert) that
expression into a smaller type like short, there would be an error.
This error is incorrect as the user actually did not have a conversion
from an offset type directly to an integer type, they went through
pointer subtraction.

Since the front-end already errors out when converting offset type
expressions to integer types, convert can just treat OFFSET_TYPE like
INTEGER_TYPE and the other types.

OK?  Bootstrapped and tested on x86_64-linux-gnu with no regressions.

Thanks,
Andrew Pinski

ChangeLog:
* convert.c (convert_to_integer): Treat OFFSET_TYPE like INTEGER_TYPE.

* g++.dg/expr/cast10.C: New test.
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